Add a wrapper to add a timestamp to a metric

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/examples_test.go

@@ -21,6 +21,7 @@ import (
 	"runtime"
 	"sort"
 	"strings"
+	"time"
 
 	dto "github.com/prometheus/client_model/go"
 	"github.com/prometheus/common/expfmt"
@@ -751,3 +752,36 @@ temperature_kelvin 4.5
 	// temperature_kelvin{location="inside"} 298.44
 	// temperature_kelvin{location="outside"} 273.14
 }
+
+func ExampleNewMetricWithTimestamp() {
+	desc := prometheus.NewDesc(
+		"temperature_kelvin",
+		"Current temperature in Kelvin.",
+		nil, nil,
+	)
+
+	// Create a constant gauge from values we got from an external
+	// temperature reporting system. Those values are reported with a slight
+	// delay, so we want to add the timestamp of the actual measurement.
+	temperatureReportedByExternalSystem := 298.15
+	timeReportedByExternalSystem := time.Date(2009, time.November, 10, 23, 0, 0, 12345678, time.UTC)
+	s := prometheus.NewMetricWithTimestamp(
+		timeReportedByExternalSystem,
+		prometheus.MustNewConstMetric(
+			desc, prometheus.GaugeValue, temperatureReportedByExternalSystem,
+		),
+	)
+
+	// Just for demonstration, let's check the state of the gauge by
+	// (ab)using its Write method (which is usually only used by Prometheus
+	// internally).
+	metric := &dto.Metric{}
+	s.Write(metric)
+	fmt.Println(proto.MarshalTextString(metric))
+
+	// Output:
+	// gauge: <
+	//   value: 298.15
+	// >
+	// timestamp_ms: 1257894000012
+}

prometheus/metric.go

@@ -15,7 +15,9 @@ package prometheus
 
 import (
 	"strings"
+	"time"
 
+	"github.com/gogo/protobuf/proto"
 	dto "github.com/prometheus/client_model/go"
 )
 
@@ -142,3 +144,31 @@ func NewInvalidMetric(desc *Desc, err error) Metric {
 func (m *invalidMetric) Desc() *Desc { return m.desc }
 
 func (m *invalidMetric) Write(*dto.Metric) error { return m.err }
+
+type timestampedMetric struct {
+	Metric
+	t time.Time
+}
+
+func (m timestampedMetric) Write(pb *dto.Metric) error {
+	e := m.Metric.Write(pb)
+	pb.TimestampMs = proto.Int64(m.t.Unix()*1000 + int64(m.t.Nanosecond()/1000000))
+	return e
+}
+
+// NewMetricWithTimestamp returns a new Metric wrapping the provided Metric in a
+// way that it has an explicit timestamp set to the provided Time. This is only
+// useful in rare cases as the timestamp of a Prometheus metric should usually
+// be set by the Prometheus server during scraping. Exceptions include mirroring
+// metrics with given timestamps from other metric
+// sources.
+//
+// NewMetricWithTimestamp works best with MustNewConstMetric,
+// MustNewConstHistogram, and MustNewConstSummary, see example.
+//
+// Currently, the exposition formats used by Prometheus are limited to
+// millisecond resolution. Thus, the provided time will be rounded down to the
+// next full millisecond value.
+func NewMetricWithTimestamp(t time.Time, m Metric) Metric {
+	return timestampedMetric{Metric: m, t: t}
+}