Registry and Metrics implement json.Marshaler

* Drop `AsMarshallable()` from the Metric interface. Use
  `json.Marshaler` and `MarshalJSON()`, and leverage JSON struct tags
  where possible.

* Add `MarshalJSON()` to Registry and remove `dumpToWriter`, which
  makes the registry handler much simpler.

In addition to simplifying some of the marshalling behavior, this also
has the nice side effect of cutting down the number of
`map[string]interface{}` instances.

prometheus/counter.go

@@ -7,6 +7,7 @@
 package prometheus
 
 import (
+	"encoding/json"
 	"fmt"
 	"sync"
 )
@@ -24,8 +25,8 @@ type Counter interface {
 }
 
 type counterVector struct {
-	labels map[string]string
-	value  float64
+	Labels map[string]string `json:"labels"`
+	Value  float64           `json:"value"`
 }
 
 func NewCounter() Counter {
@@ -49,11 +50,11 @@ func (metric *counter) Set(labels map[string]string, value float64) float64 {
 
 	signature := labelsToSignature(labels)
 	if original, ok := metric.values[signature]; ok {
-		original.value = value
+		original.Value = value
 	} else {
 		metric.values[signature] = &counterVector{
-			labels: labels,
-			value:  value,
+			Labels: labels,
+			Value:  value,
 		}
 	}
 
@@ -65,8 +66,8 @@ func (metric *counter) ResetAll() {
 	defer metric.mutex.Unlock()
 
 	for key, value := range metric.values {
-		for label := range value.labels {
-			delete(value.labels, label)
+		for label := range value.Labels {
+			delete(value.Labels, label)
 		}
 		delete(metric.values, key)
 	}
@@ -91,11 +92,11 @@ func (metric *counter) IncrementBy(labels map[string]string, value float64) floa
 
 	signature := labelsToSignature(labels)
 	if original, ok := metric.values[signature]; ok {
-		original.value += value
+		original.Value += value
 	} else {
 		metric.values[signature] = &counterVector{
-			labels: labels,
-			value:  value,
+			Labels: labels,
+			Value:  value,
 		}
 	}
 
@@ -116,11 +117,11 @@ func (metric *counter) DecrementBy(labels map[string]string, value float64) floa
 
 	signature := labelsToSignature(labels)
 	if original, ok := metric.values[signature]; ok {
-		original.value -= value
+		original.Value -= value
 	} else {
 		metric.values[signature] = &counterVector{
-			labels: labels,
-			value:  -1 * value,
+			Labels: labels,
+			Value:  -1 * value,
 		}
 	}
 
@@ -131,20 +132,18 @@ func (metric *counter) Decrement(labels map[string]string) float64 {
 	return metric.DecrementBy(labels, 1)
 }
 
-func (metric counter) AsMarshallable() map[string]interface{} {
+func (metric counter) MarshalJSON() ([]byte, error) {
 	metric.mutex.RLock()
 	defer metric.mutex.RUnlock()
 
-	values := make([]map[string]interface{}, 0, len(metric.values))
+	values := make([]*counterVector, 0, len(metric.values))
+
 	for _, value := range metric.values {
-		values = append(values, map[string]interface{}{
-			labelsKey: value.labels,
-			valueKey:  value.value,
-		})
+		values = append(values, value)
 	}
 
-	return map[string]interface{}{
+	return json.Marshal(map[string]interface{}{
 		valueKey: values,
 		typeKey:  counterTypeValue,
-	}
+	})
 }

prometheus/counter_test.go

@@ -193,9 +193,7 @@ func testCounter(t tester) {
 			step(counter)
 		}
 
-		marshallable := counter.AsMarshallable()
-
-		bytes, err := json.Marshal(marshallable)
+		bytes, err := json.Marshal(counter)
 		if err != nil {
 			t.Errorf("%d. could not marshal into JSON %s", i, err)
 			continue

prometheus/gauge.go

@@ -7,6 +7,7 @@
 package prometheus
 
 import (
+	"encoding/json"
 	"fmt"
 	"sync"
 )
@@ -21,8 +22,8 @@ type Gauge interface {
 }
 
 type gaugeVector struct {
-	labels map[string]string
-	value  float64
+	Labels map[string]string `json:"labels"`
+	Value  float64           `json:"value"`
 }
 
 func NewGauge() Gauge {
@@ -56,11 +57,11 @@ func (metric *gauge) Set(labels map[string]string, value float64) float64 {
 	signature := labelsToSignature(labels)
 
 	if original, ok := metric.values[signature]; ok {
-		original.value = value
+		original.Value = value
 	} else {
 		metric.values[signature] = &gaugeVector{
-			labels: labels,
-			value:  value,
+			Labels: labels,
+			Value:  value,
 		}
 	}
 
@@ -72,27 +73,24 @@ func (metric *gauge) ResetAll() {
 	defer metric.mutex.Unlock()
 
 	for key, value := range metric.values {
-		for label := range value.labels {
-			delete(value.labels, label)
+		for label := range value.Labels {
+			delete(value.Labels, label)
 		}
 		delete(metric.values, key)
 	}
 }
 
-func (metric gauge) AsMarshallable() map[string]interface{} {
+func (metric gauge) MarshalJSON() ([]byte, error) {
 	metric.mutex.RLock()
 	defer metric.mutex.RUnlock()
 
-	values := make([]map[string]interface{}, 0, len(metric.values))
+	values := make([]*gaugeVector, 0, len(metric.values))
 	for _, value := range metric.values {
-		values = append(values, map[string]interface{}{
-			labelsKey: value.labels,
-			valueKey:  value.value,
-		})
+		values = append(values, value)
 	}
 
-	return map[string]interface{}{
+	return json.Marshal(map[string]interface{}{
 		typeKey:  gaugeTypeValue,
 		valueKey: values,
-	}
+	})
 }

prometheus/gauge_test.go

@@ -109,9 +109,7 @@ func testGauge(t tester) {
 			step(gauge)
 		}
 
-		marshallable := gauge.AsMarshallable()
-
-		bytes, err := json.Marshal(marshallable)
+		bytes, err := json.Marshal(gauge)
 		if err != nil {
 			t.Errorf("%d. could not marshal into JSON %s", i, err)
 			continue

prometheus/histogram.go

@@ -8,6 +8,7 @@ package prometheus
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"math"
 	"strconv"
@@ -238,29 +239,30 @@ func formatFloat(value float64) string {
 	return strconv.FormatFloat(value, floatFormat, floatPrecision, floatBitCount)
 }
 
-func (h histogram) AsMarshallable() map[string]interface{} {
+func (h histogram) MarshalJSON() ([]byte, error) {
 	h.mutex.RLock()
 	defer h.mutex.RUnlock()
 
-	result := make(map[string]interface{}, 2)
-	result[typeKey] = histogramTypeValue
 	values := make([]map[string]interface{}, 0, len(h.values))
 
 	for signature, value := range h.values {
-		metricContainer := map[string]interface{}{}
-		metricContainer[labelsKey] = value.labels
-		intermediate := map[string]interface{}{}
+		percentiles := make(map[string]float64, len(h.reportablePercentiles))
+
 		for _, percentile := range h.reportablePercentiles {
 			formatted := formatFloat(percentile)
-			intermediate[formatted] = h.percentile(signature, percentile)
-		}
-		metricContainer[valueKey] = intermediate
-		values = append(values, metricContainer)
+			percentiles[formatted] = h.percentile(signature, percentile)
 		}
 
-	result[valueKey] = values
+		values = append(values, map[string]interface{}{
+			labelsKey: value.labels,
+			valueKey:  percentiles,
+		})
+	}
 
-	return result
+	return json.Marshal(map[string]interface{}{
+		typeKey:  histogramTypeValue,
+		valueKey: values,
+	})
 }
 
 func (h *histogram) ResetAll() {

prometheus/metric.go

@@ -6,10 +6,12 @@
 
 package prometheus
 
+import "encoding/json"
+
 // A Metric is something that can be exposed via the registry framework.
 type Metric interface {
-	// Produce a JSON-consumable representation of the metric.
-	AsMarshallable() map[string]interface{}
+	// Produce a JSON representation of the metric.
+	json.Marshaler
 	// Reset the parent metrics and delete all child metrics.
 	ResetAll()
 	// Produce a human-consumable representation of the metric.

prometheus/registry.go

@@ -43,9 +43,9 @@ var (
 // container represents a top-level registered metric that encompasses its
 // static metadata.
 type container struct {
-	baseLabels map[string]string
-	docstring  string
-	metric     Metric
+	BaseLabels map[string]string `json:"baseLabels"`
+	Docstring  string            `json:"docstring"`
+	Metric     Metric            `json:"metric"`
 	name       string
 }
 
@@ -82,6 +82,24 @@ func Register(name, docstring string, baseLabels map[string]string, metric Metri
 	return DefaultRegistry.Register(name, docstring, baseLabels, metric)
 }
 
+// Implements json.Marshaler
+func (r registry) MarshalJSON() (_ []byte, err error) {
+	metrics := make([]interface{}, 0, len(r.signatureContainers))
+
+	keys := make([]string, 0, len(metrics))
+	for key := range r.signatureContainers {
+		keys = append(keys, key)
+	}
+
+	sort.Strings(keys)
+
+	for _, key := range keys {
+		metrics = append(metrics, r.signatureContainers[key])
+	}
+
+	return json.Marshal(metrics)
+}
+
 // isValidCandidate returns true if the candidate is acceptable for use.  In the
 // event of any apparent incorrect use it will report the problem, invalidate
 // the candidate, or outright abort.
@@ -125,7 +143,7 @@ func (r registry) isValidCandidate(name string, baseLabels map[string]string) (s
 	if useAggressiveSanityChecks {
 		for _, container := range r.signatureContainers {
 			if container.name == name {
-				err = fmt.Errorf("metric named %s with baseLabels %s is already registered as %s and risks causing confusion", name, baseLabels, container.baseLabels)
+				err = fmt.Errorf("metric named %s with baseLabels %s is already registered as %s and risks causing confusion", name, baseLabels, container.BaseLabels)
 				if abortOnMisuse {
 					panic(err)
 				} else if debugRegistration {
@@ -154,9 +172,9 @@ func (r registry) Register(name, docstring string, baseLabels map[string]string,
 	}
 
 	r.signatureContainers[signature] = container{
-		baseLabels: baseLabels,
-		docstring:  docstring,
-		metric:     metric,
+		BaseLabels: baseLabels,
+		Docstring:  docstring,
+		Metric:     metric,
 		name:       name,
 	}
 
@@ -194,61 +212,6 @@ func (register registry) YieldBasicAuthExporter(username, password string) http.
 	})
 }
 
-func (registry registry) dumpToWriter(writer io.Writer) (err error) {
-	defer func() {
-		if err != nil {
-			dumpErrorCount.Increment(nil)
-		}
-	}()
-
-	numberOfMetrics := len(registry.signatureContainers)
-	keys := make([]string, 0, numberOfMetrics)
-	for key := range registry.signatureContainers {
-		keys = append(keys, key)
-	}
-
-	sort.Strings(keys)
-
-	_, err = writer.Write([]byte("["))
-	if err != nil {
-		return
-	}
-
-	index := 0
-
-	for _, key := range keys {
-		container := registry.signatureContainers[key]
-		intermediate := map[string]interface{}{
-			baseLabelsKey: container.baseLabels,
-			docstringKey:  container.docstring,
-			metricKey:     container.metric.AsMarshallable(),
-		}
-		marshaled, err := json.Marshal(intermediate)
-		if err != nil {
-			marshalErrorCount.Increment(nil)
-			index++
-			continue
-		}
-
-		if index > 0 && index < numberOfMetrics {
-			_, err = writer.Write([]byte(","))
-			if err != nil {
-				return err
-			}
-		}
-
-		_, err = writer.Write(marshaled)
-		if err != nil {
-			return err
-		}
-		index++
-	}
-
-	_, err = writer.Write([]byte("]"))
-
-	return
-}
-
 // decorateWriter annotates the response writer to handle any other behaviors
 // that might be beneficial to the client---e.g., GZIP encoding.
 func decorateWriter(request *http.Request, writer http.ResponseWriter) io.Writer {
@@ -286,8 +249,7 @@ func (registry registry) Handler() http.HandlerFunc {
 				defer closer.Close()
 			}
 
-			registry.dumpToWriter(writer)
-
+			json.NewEncoder(writer).Encode(registry)
 		} else {
 			w.WriteHeader(http.StatusNotFound)
 		}

prometheus/registry_test.go

@@ -8,6 +8,7 @@ package prometheus
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
@@ -305,15 +306,14 @@ func testDumpToWriter(t tester) {
 			}
 		}
 
-		actual := &bytes.Buffer{}
+		actual, err := json.Marshal(registry)
 
-		err := registry.dumpToWriter(actual)
 		if err != nil {
 			t.Errorf("%d. encountered error while dumping %s", i, err)
 		}
 
-		if !bytes.Equal(scenario.out, actual.Bytes()) {
-			t.Errorf("%d. expected %q for dumping, got %q", i, scenario.out, actual.Bytes())
+		if !bytes.Equal(scenario.out, actual) {
+			t.Errorf("%d. expected %q for dumping, got %q", i, scenario.out, actual)
 		}
 	}
 }

prometheus/telemetry.go

@@ -15,9 +15,6 @@ import (
 // exposed if the DefaultRegistry's exporter is hooked into the HTTP request
 // handler.
 var (
-	marshalErrorCount = NewCounter()
-	dumpErrorCount    = NewCounter()
-
 	requestCount          = NewCounter()
 	requestLatencyBuckets = LogarithmicSizedBucketsFor(0, 1000)
 	requestLatency        = NewHistogram(&HistogramSpecification{