Add support for exemplars on constHistogram

Co-authored-by: William Perron <william.perron@shopify.com>
Signed-off-by: William Perron <william.perron@shopify.com>

prometheus/examples_test.go

@@ -16,6 +16,7 @@ package prometheus_test
 import (
 	"bytes"
 	"fmt"
+	"google.golang.org/protobuf/types/known/timestamppb"
 	"math"
 	"net/http"
 	"runtime"
@@ -549,11 +550,27 @@ func ExampleNewConstHistogram() {
 		prometheus.Labels{"owner": "example"},
 	)
 
+	var exemplars []*dto.Exemplar
+	n := "testName"
+	v := "testVal"
+	lp := dto.LabelPair{Name: &n, Value: &v}
+	var labelPairs []*dto.LabelPair
+	labelPairs = append(labelPairs, &lp)
+	val := float64(42)
+	t, _ := time.Parse("unix", "Mon Jan _2 15:04:05 MST 2006")
+	ts := timestamppb.New(t)
+
+	for i := 0; i < 4; i++ {
+		e := dto.Exemplar{Label: labelPairs, Value: &val, Timestamp: ts}
+		exemplars = append(exemplars, &e)
+	}
+
 	// Create a constant histogram from values we got from a 3rd party telemetry system.
-	h := prometheus.MustNewConstHistogram(
+	h := prometheus.MustNewConstHistogramWithExemplar(
 		desc,
 		4711, 403.34,
 		map[float64]uint64{25: 121, 50: 2403, 100: 3221, 200: 4233},
+		exemplars,
 		"200", "get",
 	)
 
@@ -583,18 +600,58 @@ func ExampleNewConstHistogram() {
 	//   bucket: <
 	//     cumulative_count: 121
 	//     upper_bound: 25
+	//     exemplar: <
+	//       label: <
+	//         name: "testName"
+	//         value: "testVal"
+	//       >
+	//       value: 42
+	//       timestamp: <
+	//         seconds: -62135596800
+	//       >
+	//     >
 	//   >
 	//   bucket: <
 	//     cumulative_count: 2403
 	//     upper_bound: 50
+	//     exemplar: <
+	//       label: <
+	//         name: "testName"
+	//         value: "testVal"
+	//       >
+	//       value: 42
+	//       timestamp: <
+	//         seconds: -62135596800
+	//       >
+	//     >
 	//   >
 	//   bucket: <
 	//     cumulative_count: 3221
 	//     upper_bound: 100
+	//     exemplar: <
+	//       label: <
+	//         name: "testName"
+	//         value: "testVal"
+	//       >
+	//       value: 42
+	//       timestamp: <
+	//         seconds: -62135596800
+	//       >
+	//     >
 	//   >
 	//   bucket: <
 	//     cumulative_count: 4233
 	//     upper_bound: 200
+	//     exemplar: <
+	//       label: <
+	//         name: "testName"
+	//         value: "testVal"
+	//       >
+	//       value: 42
+	//       timestamp: <
+	//         seconds: -62135596800
+	//       >
+	//     >
 	//   >
 	// >
 }

prometheus/histogram.go

@@ -573,6 +573,7 @@ type constHistogram struct {
 	sum        float64
 	buckets    map[float64]uint64
 	labelPairs []*dto.LabelPair
+	exemplars  []*dto.Exemplar
 }
 
 func (h *constHistogram) Desc() *Desc {
@@ -585,7 +586,6 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 
 	his.SampleCount = proto.Uint64(h.count)
 	his.SampleSum = proto.Float64(h.sum)
-
 	for upperBound, count := range h.buckets {
 		buckets = append(buckets, &dto.Bucket{
 			CumulativeCount: proto.Uint64(count),
@@ -596,6 +596,13 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 	if len(buckets) > 0 {
 		sort.Sort(buckSort(buckets))
 	}
+
+	if len(h.exemplars) > 0 {
+		for i := 0; i < len(buckets); i++ {
+			buckets[i].Exemplar = h.exemplars[i]
+		}
+	}
+
 	his.Bucket = buckets
 
 	out.Histogram = his
@@ -604,6 +611,13 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 	return nil
 }
 
+func (h *constHistogram) GetExemplars() []*dto.Exemplar {
+	if h != nil {
+		return h.exemplars
+	}
+	return nil
+}
+
 // NewConstHistogram returns a metric representing a Prometheus histogram with
 // fixed values for the count, sum, and bucket counts. As those parameters
 // cannot be changed, the returned value does not implement the Histogram
@@ -617,6 +631,7 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 //
 // NewConstHistogram returns an error if the length of labelValues is not
 // consistent with the variable labels in Desc or if Desc is invalid.
+
 func NewConstHistogram(
 	desc *Desc,
 	count uint64,
@@ -655,6 +670,48 @@ func MustNewConstHistogram(
 	return m
 }
 
+func NewConstHistogramWithExemplar(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	buckets map[float64]uint64,
+	exemplars []*dto.Exemplar,
+	labelValues ...string,
+
+) (Metric, error) {
+	if desc.err != nil {
+		return nil, desc.err
+	}
+	if err := validateLabelValues(labelValues, len(desc.variableLabels)); err != nil {
+		return nil, err
+	}
+	return &constHistogram{
+		desc:       desc,
+		count:      count,
+		sum:        sum,
+		buckets:    buckets,
+		exemplars:  exemplars,
+		labelPairs: MakeLabelPairs(desc, labelValues),
+	}, nil
+}
+
+// MustNewConstHistogram is a version of NewConstHistogram that panics where
+// NewConstHistogram would have returned an error.
+func MustNewConstHistogramWithExemplar(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	buckets map[float64]uint64,
+	exemplars []*dto.Exemplar,
+	labelValues ...string,
+) Metric {
+	m, err := NewConstHistogramWithExemplar(desc, count, sum, buckets, exemplars, labelValues...)
+	if err != nil {
+		panic(err)
+	}
+	return m
+}
+
 type buckSort []*dto.Bucket
 
 func (s buckSort) Len() int {