Add exemplars for native histograms

prometheus/metric.go

@@ -187,6 +187,12 @@ func (m *withExemplarsMetric) Write(pb *dto.Metric) error {
 		pb.Counter.Exemplar = m.exemplars[len(m.exemplars)-1]
 	case pb.Histogram != nil:
 		for _, e := range m.exemplars {
+			if pb.Histogram.Schema != nil {
+				if *pb.Histogram.Schema > math.MinInt32 && e.GetTimestamp() != nil {
+					pb.Histogram.Exemplars = append(pb.Histogram.Exemplars, e)
+				}
+				continue
+			}
 			// pb.Histogram.Bucket are sorted by UpperBound.
 			i := sort.Search(len(pb.Histogram.Bucket), func(i int) bool {
 				return pb.Histogram.Bucket[i].GetUpperBound() >= e.GetValue()
@@ -227,6 +233,7 @@ type Exemplar struct {
 // Only last applicable exemplar is injected from the list.
 // For example for Counter it means last exemplar is injected.
 // For Histogram, it means last applicable exemplar for each bucket is injected.
+// Note that for a nativeHistogram, all valid exemplars are injected.
 //
 // NewMetricWithExemplars works best with MustNewConstMetric and
 // MustNewConstHistogram, see example.

prometheus/metric_test.go

@@ -16,10 +16,12 @@ package prometheus
 import (
 	"math"
 	"testing"
+	"time"
 
 	dto "github.com/prometheus/client_model/go"
 
 	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/known/timestamppb"
 )
 
 func TestBuildFQName(t *testing.T) {
@@ -90,3 +92,29 @@ func TestWithExemplarsMetric(t *testing.T) {
 		}
 	})
 }
+
+func TestWithExemplarsNativeHistogramMetric(t *testing.T) {
+	t.Run("histogram", func(t *testing.T) {
+		// Create a constant histogram from values we got from a 3rd party telemetry system.
+		h := MustNewConstNativeHistogram(
+			NewDesc("http_request_duration_seconds", "A histogram of the HTTP request durations.", nil, nil),
+			10, 12.1, map[int]int64{1: 7, 2: 1, 3: 2}, map[int]int64{}, 0, 2, 0.2, time.Date(
+				2009, 11, 17, 20, 34, 58, 651387237, time.UTC))
+		m := &withExemplarsMetric{Metric: h, exemplars: []*dto.Exemplar{
+			{Value: proto.Float64(2000.0), Timestamp: timestamppb.New(time.Date(2009, 11, 17, 20, 34, 58, 3243244, time.UTC))},
+		}}
+		metric := dto.Metric{}
+		if err := m.Write(&metric); err != nil {
+			t.Fatal(err)
+		}
+		if want, got := 1, len(metric.GetHistogram().Exemplars); want != got {
+			t.Errorf("want %v, got %v", want, got)
+		}
+
+		for _, b := range metric.GetHistogram().Bucket {
+			if b.Exemplar != nil {
+				t.Error("Not expecting exemplar for bucket")
+			}
+		}
+	})
+}