Add native histograms

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Apply suggestions from code review

Co-authored-by: Arthur Silva Sens <arthursens2005@gmail.com>
Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Fix references and existing tests

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Review comments

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

fmt

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

validation

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Count validation

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Validation

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Review comments

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Space formatting

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Review comments

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Rename SyncMaptoMap -> SyncMapToMap

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Remove exemplars from parameters for constNativeHistogram function

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Update prometheus/histogram.go

Co-authored-by: George Krajcsovits <krajorama@users.noreply.github.com>
Signed-off-by: Shivanth MP <shivanth.metchem@deliveryhero.com>

Update prometheus/histogram.go

Co-authored-by: George Krajcsovits <krajorama@users.noreply.github.com>
Signed-off-by: Shivanth MP <shivanth.metchem@deliveryhero.com>

Update prometheus/histogram.go

Co-authored-by: George Krajcsovits <krajorama@users.noreply.github.com>
Signed-off-by: Shivanth MP <shivanth.metchem@deliveryhero.com>

Update prometheus/histogram.go

Co-authored-by: Björn Rabenstein <github@rabenste.in>
Signed-off-by: Shivanth MP <shivanth.metchem@deliveryhero.com>

Review comments

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

Lint fix

Signed-off-by: Shivanth <shivanth.metchem@deliveryhero.com>

prometheus/histogram.go

@@ -14,6 +14,7 @@
 package prometheus
 
 import (
+	"errors"
 	"fmt"
 	"math"
 	"runtime"
@@ -28,6 +29,11 @@ import (
 	"google.golang.org/protobuf/types/known/timestamppb"
 )
 
+const (
+	nativeHistogramSchemaMaximum = 8
+	nativeHistogramSchemaMinimum = -4
+)
+
 // nativeHistogramBounds for the frac of observed values. Only relevant for
 // schema > 0. The position in the slice is the schema. (0 is never used, just
 // here for convenience of using the schema directly as the index.)
@@ -1460,9 +1466,9 @@ func pickSchema(bucketFactor float64) int32 {
 	floor := math.Floor(math.Log2(math.Log2(bucketFactor)))
 	switch {
 	case floor <= -8:
-		return 8
+		return nativeHistogramSchemaMaximum
 	case floor >= 4:
-		return -4
+		return nativeHistogramSchemaMinimum
 	default:
 		return -int32(floor)
 	}
@@ -1851,3 +1857,196 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
 		n.exemplars = append(n.exemplars[:nIdx], append([]*dto.Exemplar{e}, append(n.exemplars[nIdx:rIdx], n.exemplars[rIdx+1:]...)...)...)
 	}
 }
+
+type constNativeHistogram struct {
+	desc *Desc
+	dto.Histogram
+	labelPairs []*dto.LabelPair
+}
+
+func validateCount(sum float64, count uint64, negativeBuckets, positiveBuckets map[int]int64, zeroBucket uint64) error {
+	var bucketPopulationSum int64
+	for _, v := range positiveBuckets {
+		bucketPopulationSum += v
+	}
+	for _, v := range negativeBuckets {
+		bucketPopulationSum += v
+	}
+	bucketPopulationSum += int64(zeroBucket)
+
+	// If the sum of observations is NaN, the number of observations must be greater or equal to the sum of all bucket counts.
+	// Otherwise, the number of observations must be equal to the sum of all bucket counts .
+
+	if math.IsNaN(sum) && bucketPopulationSum > int64(count) ||
+		!math.IsNaN(sum) && bucketPopulationSum != int64(count) {
+		return errors.New("the sum of all bucket populations exceeds the count of observations")
+	}
+	return nil
+}
+
+// NewConstNativeHistogram returns a metric representing a Prometheus native histogram with
+// fixed values for the count, sum, and positive/negative/zero bucket counts. As those parameters
+// cannot be changed, the returned value does not implement the Histogram
+// interface (but only the Metric interface). Users of this package will not
+// have much use for it in regular operations. However, when implementing custom
+// OpenTelemetry Collectors, it is useful as a throw-away metric that is generated on the fly
+// to send it to Prometheus in the Collect method.
+//
+// zeroBucket counts all (positive and negative)
+// observations in the zero bucket (with an absolute value less or equal
+// the current threshold).
+// positiveBuckets and negativeBuckets are separate maps for negative and positive
+// observations. The map's value is an int64, counting observations in
+// that bucket. The map's key is the
+// index of the bucket according to the used
+// Schema. Index 0 is for an upper bound of 1 in positive buckets and for a lower bound of -1 in negative buckets.
+// NewConstNativeHistogram returns an error if
+//   - the length of labelValues is not consistent with the variable labels in Desc or if Desc is invalid.
+//   - the schema passed is not between 8 and -4
+//   - the sum of counts in all buckets including the zero bucket does not equal the count if sum is not NaN (or exceeds the count if sum is NaN)
+//
+// See https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#exponential-histograms for more details about the conversion from OTel to Prometheus.
+func NewConstNativeHistogram(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	positiveBuckets, negativeBuckets map[int]int64,
+	zeroBucket uint64,
+	schema int32,
+	zeroThreshold float64,
+	createdTimestamp time.Time,
+	labelValues ...string,
+) (Metric, error) {
+	if desc.err != nil {
+		return nil, desc.err
+	}
+	if err := validateLabelValues(labelValues, len(desc.variableLabels.names)); err != nil {
+		return nil, err
+	}
+	if schema > nativeHistogramSchemaMaximum || schema < nativeHistogramSchemaMinimum {
+		return nil, errors.New("invalid native histogram schema")
+	}
+	if err := validateCount(sum, count, negativeBuckets, positiveBuckets, zeroBucket); err != nil {
+		return nil, err
+	}
+
+	NegativeSpan, NegativeDelta := makeBucketsFromMap(negativeBuckets)
+	PositiveSpan, PositiveDelta := makeBucketsFromMap(positiveBuckets)
+	ret := &constNativeHistogram{
+		desc: desc,
+		Histogram: dto.Histogram{
+			CreatedTimestamp: timestamppb.New(createdTimestamp),
+			Schema:           &schema,
+			ZeroThreshold:    &zeroThreshold,
+			SampleCount:      &count,
+			SampleSum:        &sum,
+
+			NegativeSpan:  NegativeSpan,
+			NegativeDelta: NegativeDelta,
+
+			PositiveSpan:  PositiveSpan,
+			PositiveDelta: PositiveDelta,
+
+			ZeroCount: proto.Uint64(zeroBucket),
+		},
+		labelPairs: MakeLabelPairs(desc, labelValues),
+	}
+	if *ret.ZeroThreshold == 0 && *ret.ZeroCount == 0 && len(ret.PositiveSpan) == 0 && len(ret.NegativeSpan) == 0 {
+		ret.PositiveSpan = []*dto.BucketSpan{{
+			Offset: proto.Int32(0),
+			Length: proto.Uint32(0),
+		}}
+	}
+	return ret, nil
+}
+
+// MustNewConstNativeHistogram is a version of NewConstNativeHistogram that panics where
+// NewConstNativeHistogram would have returned an error.
+func MustNewConstNativeHistogram(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	positiveBuckets, negativeBuckets map[int]int64,
+	zeroBucket uint64,
+	nativeHistogramSchema int32,
+	nativeHistogramZeroThreshold float64,
+	createdTimestamp time.Time,
+	labelValues ...string,
+) Metric {
+	nativehistogram, err := NewConstNativeHistogram(desc,
+		count,
+		sum,
+		positiveBuckets,
+		negativeBuckets,
+		zeroBucket,
+		nativeHistogramSchema,
+		nativeHistogramZeroThreshold,
+		createdTimestamp,
+		labelValues...)
+	if err != nil {
+		panic(err)
+	}
+	return nativehistogram
+}
+
+func (h *constNativeHistogram) Desc() *Desc {
+	return h.desc
+}
+
+func (h *constNativeHistogram) Write(out *dto.Metric) error {
+	out.Histogram = &h.Histogram
+	out.Label = h.labelPairs
+	return nil
+}
+
+func makeBucketsFromMap(buckets map[int]int64) ([]*dto.BucketSpan, []int64) {
+	if len(buckets) == 0 {
+		return nil, nil
+	}
+	var ii []int
+	for k := range buckets {
+		ii = append(ii, k)
+	}
+	sort.Ints(ii)
+
+	var (
+		spans     []*dto.BucketSpan
+		deltas    []int64
+		prevCount int64
+		nextI     int
+	)
+
+	appendDelta := func(count int64) {
+		*spans[len(spans)-1].Length++
+		deltas = append(deltas, count-prevCount)
+		prevCount = count
+	}
+
+	for n, i := range ii {
+		count := buckets[i]
+		// Multiple spans with only small gaps in between are probably
+		// encoded more efficiently as one larger span with a few empty
+		// buckets. Needs some research to find the sweet spot. For now,
+		// we assume that gaps of one or two buckets should not create
+		// a new span.
+		iDelta := int32(i - nextI)
+		if n == 0 || iDelta > 2 {
+			// We have to create a new span, either because we are
+			// at the very beginning, or because we have found a gap
+			// of more than two buckets.
+			spans = append(spans, &dto.BucketSpan{
+				Offset: proto.Int32(iDelta),
+				Length: proto.Uint32(0),
+			})
+		} else {
+			// We have found a small gap (or no gap at all).
+			// Insert empty buckets as needed.
+			for j := int32(0); j < iDelta; j++ {
+				appendDelta(0)
+			}
+		}
+		appendDelta(count)
+		nextI = i + 1
+	}
+	return spans, deltas
+}

prometheus/histogram_test.go

@@ -25,11 +25,11 @@ import (
 	"testing/quick"
 	"time"
 
-	"github.com/prometheus/client_golang/prometheus/internal"
-
 	dto "github.com/prometheus/client_model/go"
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/types/known/timestamppb"
+
+	"github.com/prometheus/client_golang/prometheus/internal"
 )
 
 func benchmarkHistogramObserve(w int, b *testing.B) {
@@ -1543,3 +1543,544 @@ func TestFindBucket(t *testing.T) {
 		}
 	}
 }
+
+func syncMapToMap(syncmap *sync.Map) (m map[int]int64) {
+	m = map[int]int64{}
+	syncmap.Range(func(key, value any) bool {
+		m[key.(int)] = *value.(*int64)
+		return true
+	})
+	return m
+}
+
+func TestConstNativeHistogram(t *testing.T) {
+	now := time.Now()
+
+	scenarios := []struct {
+		name             string
+		observations     []float64 // With simulated interval of 1m.
+		factor           float64
+		zeroThreshold    float64
+		maxBuckets       uint32
+		minResetDuration time.Duration
+		maxZeroThreshold float64
+		want             *dto.Histogram
+	}{
+		{
+			name:   "no observations",
+			factor: 1.1,
+			want: &dto.Histogram{
+				SampleCount:      proto.Uint64(0),
+				SampleSum:        proto.Float64(0),
+				Schema:           proto.Int32(3),
+				ZeroThreshold:    proto.Float64(2.938735877055719e-39),
+				ZeroCount:        proto.Uint64(0),
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:          "no observations and zero threshold of zero resulting in no-op span",
+			factor:        1.1,
+			zeroThreshold: NativeHistogramZeroThresholdZero,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(0),
+				SampleSum:     proto.Float64(0),
+				Schema:        proto.Int32(3),
+				ZeroThreshold: proto.Float64(0),
+				ZeroCount:     proto.Uint64(0),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(0)},
+				},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "factor 1.1 results in schema 3",
+			observations: []float64{0, 1, 2, 3},
+			factor:       1.1,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(4),
+				SampleSum:     proto.Float64(6),
+				Schema:        proto.Int32(3),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(1)},
+					{Offset: proto.Int32(7), Length: proto.Uint32(1)},
+					{Offset: proto.Int32(4), Length: proto.Uint32(1)},
+				},
+				PositiveDelta:    []int64{1, 0, 0},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "factor 1.2 results in schema 2",
+			observations: []float64{0, 1, 1.2, 1.4, 1.8, 2},
+			factor:       1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(6),
+				SampleSum:     proto.Float64(7.4),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				PositiveDelta:    []int64{1, -1, 2, -2, 2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name: "factor 4 results in schema -1",
+			observations: []float64{
+				0.0156251, 0.0625, // Bucket -2: (0.015625, 0.0625)
+				0.1, 0.25, // Bucket -1: (0.0625, 0.25]
+				0.5, 1, // Bucket 0: (0.25, 1]
+				1.5, 2, 3, 3.5, // Bucket 1: (1, 4]
+				5, 6, 7, // Bucket 2: (4, 16]
+				33.33, // Bucket 3: (16, 64]
+			},
+			factor: 4,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(14),
+				SampleSum:     proto.Float64(63.2581251),
+				Schema:        proto.Int32(-1),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(0),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(-2), Length: proto.Uint32(6)},
+				},
+				PositiveDelta:    []int64{2, 0, 0, 2, -1, -2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name: "factor 17 results in schema -2",
+			observations: []float64{
+				0.0156251, 0.0625, // Bucket -1: (0.015625, 0.0625]
+				0.1, 0.25, 0.5, 1, // Bucket 0: (0.0625, 1]
+				1.5, 2, 3, 3.5, 5, 6, 7, // Bucket 1: (1, 16]
+				33.33, // Bucket 2: (16, 256]
+			},
+			factor: 17,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(14),
+				SampleSum:     proto.Float64(63.2581251),
+				Schema:        proto.Int32(-2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(0),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(-1), Length: proto.Uint32(4)},
+				},
+				PositiveDelta:    []int64{2, 2, 3, -6},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "negative buckets",
+			observations: []float64{0, -1, -1.2, -1.4, -1.8, -2},
+			factor:       1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(6),
+				SampleSum:     proto.Float64(-7.4),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				NegativeDelta:    []int64{1, -1, 2, -2, 2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "negative and positive buckets",
+			observations: []float64{0, -1, -1.2, -1.4, -1.8, -2, 1, 1.2, 1.4, 1.8, 2},
+			factor:       1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(11),
+				SampleSum:     proto.Float64(0),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				NegativeDelta: []int64{1, -1, 2, -2, 2},
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				PositiveDelta:    []int64{1, -1, 2, -2, 2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:          "wide zero bucket",
+			observations:  []float64{0, -1, -1.2, -1.4, -1.8, -2, 1, 1.2, 1.4, 1.8, 2},
+			factor:        1.2,
+			zeroThreshold: 1.4,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(11),
+				SampleSum:     proto.Float64(0),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(1.4),
+				ZeroCount:     proto.Uint64(7),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(4), Length: proto.Uint32(1)},
+				},
+				NegativeDelta: []int64{2},
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(4), Length: proto.Uint32(1)},
+				},
+				PositiveDelta:    []int64{2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "NaN observation",
+			observations: []float64{0, 1, 1.2, 1.4, 1.8, 2, math.NaN()},
+			factor:       1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(7),
+				SampleSum:     proto.Float64(math.NaN()),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				PositiveDelta:    []int64{1, -1, 2, -2, 2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "+Inf observation",
+			observations: []float64{0, 1, 1.2, 1.4, 1.8, 2, math.Inf(+1)},
+			factor:       1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(7),
+				SampleSum:     proto.Float64(math.Inf(+1)),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+					{Offset: proto.Int32(4092), Length: proto.Uint32(1)},
+				},
+				PositiveDelta:    []int64{1, -1, 2, -2, 2, -1},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "-Inf observation",
+			observations: []float64{0, 1, 1.2, 1.4, 1.8, 2, math.Inf(-1)},
+			factor:       1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(7),
+				SampleSum:     proto.Float64(math.Inf(-1)),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(4097), Length: proto.Uint32(1)},
+				},
+				NegativeDelta: []int64{1},
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				PositiveDelta:    []int64{1, -1, 2, -2, 2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "limited buckets but nothing triggered",
+			observations: []float64{0, 1, 1.2, 1.4, 1.8, 2},
+			factor:       1.2,
+			maxBuckets:   4,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(6),
+				SampleSum:     proto.Float64(7.4),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				PositiveDelta:    []int64{1, -1, 2, -2, 2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "buckets limited by halving resolution",
+			observations: []float64{0, 1, 1.1, 1.2, 1.4, 1.8, 2, 3},
+			factor:       1.2,
+			maxBuckets:   4,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(8),
+				SampleSum:     proto.Float64(11.5),
+				Schema:        proto.Int32(1),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				PositiveDelta:    []int64{1, 2, -1, -2, 1},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:             "buckets limited by widening the zero bucket",
+			observations:     []float64{0, 1, 1.1, 1.2, 1.4, 1.8, 2, 3},
+			factor:           1.2,
+			maxBuckets:       4,
+			maxZeroThreshold: 1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(8),
+				SampleSum:     proto.Float64(11.5),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(1),
+				ZeroCount:     proto.Uint64(2),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(1), Length: proto.Uint32(7)},
+				},
+				PositiveDelta:    []int64{1, 1, -2, 2, -2, 0, 1},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:             "buckets limited by widening the zero bucket twice",
+			observations:     []float64{0, 1, 1.1, 1.2, 1.4, 1.8, 2, 3, 4},
+			factor:           1.2,
+			maxBuckets:       4,
+			maxZeroThreshold: 1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(9),
+				SampleSum:     proto.Float64(15.5),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(1.189207115002721),
+				ZeroCount:     proto.Uint64(3),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(2), Length: proto.Uint32(7)},
+				},
+				PositiveDelta:    []int64{2, -2, 2, -2, 0, 1, 0},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:             "buckets limited by reset",
+			observations:     []float64{0, 1, 1.1, 1.2, 1.4, 1.8, 2, 3, 4},
+			factor:           1.2,
+			maxBuckets:       4,
+			maxZeroThreshold: 1.2,
+			minResetDuration: 5 * time.Minute,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(2),
+				SampleSum:     proto.Float64(7),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(0),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(7), Length: proto.Uint32(2)},
+				},
+				PositiveDelta:    []int64{1, 0},
+				CreatedTimestamp: timestamppb.New(now.Add(8 * time.Minute)), // We expect reset to happen after 8 observations.
+			},
+		},
+		{
+			name:         "limited buckets but nothing triggered, negative observations",
+			observations: []float64{0, -1, -1.2, -1.4, -1.8, -2},
+			factor:       1.2,
+			maxBuckets:   4,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(6),
+				SampleSum:     proto.Float64(-7.4),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				NegativeDelta:    []int64{1, -1, 2, -2, 2},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:         "buckets limited by halving resolution, negative observations",
+			observations: []float64{0, -1, -1.1, -1.2, -1.4, -1.8, -2, -3},
+			factor:       1.2,
+			maxBuckets:   4,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(8),
+				SampleSum:     proto.Float64(-11.5),
+				Schema:        proto.Int32(1),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(1),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(0), Length: proto.Uint32(5)},
+				},
+				NegativeDelta:    []int64{1, 2, -1, -2, 1},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:             "buckets limited by widening the zero bucket, negative observations",
+			observations:     []float64{0, -1, -1.1, -1.2, -1.4, -1.8, -2, -3},
+			factor:           1.2,
+			maxBuckets:       4,
+			maxZeroThreshold: 1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(8),
+				SampleSum:     proto.Float64(-11.5),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(1),
+				ZeroCount:     proto.Uint64(2),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(1), Length: proto.Uint32(7)},
+				},
+				NegativeDelta:    []int64{1, 1, -2, 2, -2, 0, 1},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:             "buckets limited by widening the zero bucket twice, negative observations",
+			observations:     []float64{0, -1, -1.1, -1.2, -1.4, -1.8, -2, -3, -4},
+			factor:           1.2,
+			maxBuckets:       4,
+			maxZeroThreshold: 1.2,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(9),
+				SampleSum:     proto.Float64(-15.5),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(1.189207115002721),
+				ZeroCount:     proto.Uint64(3),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(2), Length: proto.Uint32(7)},
+				},
+				NegativeDelta:    []int64{2, -2, 2, -2, 0, 1, 0},
+				CreatedTimestamp: timestamppb.New(now),
+			},
+		},
+		{
+			name:             "buckets limited by reset, negative observations",
+			observations:     []float64{0, -1, -1.1, -1.2, -1.4, -1.8, -2, -3, -4},
+			factor:           1.2,
+			maxBuckets:       4,
+			maxZeroThreshold: 1.2,
+			minResetDuration: 5 * time.Minute,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(2),
+				SampleSum:     proto.Float64(-7),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(0),
+				NegativeSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(7), Length: proto.Uint32(2)},
+				},
+				NegativeDelta:    []int64{1, 0},
+				CreatedTimestamp: timestamppb.New(now.Add(8 * time.Minute)), // We expect reset to happen after 8 observations.
+			},
+		},
+		{
+			name:             "buckets limited by halving resolution, then reset",
+			observations:     []float64{0, 1, 1.1, 1.2, 1.4, 1.8, 2, 5, 5.1, 3, 4},
+			factor:           1.2,
+			maxBuckets:       4,
+			minResetDuration: 9 * time.Minute,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(3),
+				SampleSum:     proto.Float64(12.1),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(0),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(7), Length: proto.Uint32(4)},
+				},
+				PositiveDelta:    []int64{1, 0, -1, 1},
+				CreatedTimestamp: timestamppb.New(now.Add(9 * time.Minute)), // We expect reset to happen after 8 minutes.
+			},
+		},
+		{
+			name:             "buckets limited by widening the zero bucket, then reset",
+			observations:     []float64{0, 1, 1.1, 1.2, 1.4, 1.8, 2, 5, 5.1, 3, 4},
+			factor:           1.2,
+			maxBuckets:       4,
+			maxZeroThreshold: 1.2,
+			minResetDuration: 9 * time.Minute,
+			want: &dto.Histogram{
+				SampleCount:   proto.Uint64(3),
+				SampleSum:     proto.Float64(12.1),
+				Schema:        proto.Int32(2),
+				ZeroThreshold: proto.Float64(2.938735877055719e-39),
+				ZeroCount:     proto.Uint64(0),
+				PositiveSpan: []*dto.BucketSpan{
+					{Offset: proto.Int32(7), Length: proto.Uint32(4)},
+				},
+				PositiveDelta:    []int64{1, 0, -1, 1},
+				CreatedTimestamp: timestamppb.New(now.Add(9 * time.Minute)), // We expect reset to happen after 8 minutes.
+			},
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			var (
+				ts         = now
+				funcToCall func()
+				whenToCall time.Duration
+			)
+
+			his := NewHistogram(HistogramOpts{
+				Name:                            "name",
+				Help:                            "help",
+				NativeHistogramBucketFactor:     s.factor,
+				NativeHistogramZeroThreshold:    s.zeroThreshold,
+				NativeHistogramMaxBucketNumber:  s.maxBuckets,
+				NativeHistogramMinResetDuration: s.minResetDuration,
+				NativeHistogramMaxZeroThreshold: s.maxZeroThreshold,
+				now:                             func() time.Time { return ts },
+				afterFunc: func(d time.Duration, f func()) *time.Timer {
+					funcToCall = f
+					whenToCall = d
+					return nil
+				},
+			})
+
+			ts = ts.Add(time.Minute)
+			for _, o := range s.observations {
+				his.Observe(o)
+				ts = ts.Add(time.Minute)
+				whenToCall -= time.Minute
+				if funcToCall != nil && whenToCall <= 0 {
+					funcToCall()
+					funcToCall = nil
+				}
+			}
+			_his := his.(*histogram)
+			n := atomic.LoadUint64(&_his.countAndHotIdx)
+			hotIdx := n >> 63
+			cold := _his.counts[hotIdx]
+			consthist, err := NewConstNativeHistogram(_his.Desc(),
+				cold.count,
+				math.Float64frombits(cold.sumBits),
+				syncMapToMap(&cold.nativeHistogramBucketsPositive),
+				syncMapToMap(&cold.nativeHistogramBucketsNegative),
+				cold.nativeHistogramZeroBucket,
+				cold.nativeHistogramSchema,
+				math.Float64frombits(cold.nativeHistogramZeroThresholdBits),
+				_his.lastResetTime,
+			)
+			if err != nil {
+				t.Fatal("unexpected error writing metric", err)
+			}
+			m2 := &dto.Metric{}
+
+			if err := consthist.Write(m2); err != nil {
+				t.Fatal("unexpected error writing metric", err)
+			}
+			got := m2.Histogram
+			if !proto.Equal(s.want, got) {
+				t.Errorf("want histogram %q, got %q", s.want, got)
+			}
+		})
+	}
+}