explicitly add +inf bucket in withExemplarsMetric (#1094)

* explicitly adding +inf bucket to withExemplarsMetric

Signed-off-by: Arun Mahendra <arun.mahendra@shopify.com>

* Update prometheus/metric_test.go

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

* Update prometheus/metric.go

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

* updated comment and removed unnecessary test

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

Author: arun-shopify

prometheus/metric.go

@@ -15,6 +15,7 @@ package prometheus
 
 import (
 	"errors"
+	"math"
 	"sort"
 	"strings"
 	"time"
@@ -184,8 +185,15 @@ func (m *withExemplarsMetric) Write(pb *dto.Metric) error {
 			if i < len(pb.Histogram.Bucket) {
 				pb.Histogram.Bucket[i].Exemplar = e
 			} else {
-				// This is not possible as last bucket is Inf.
-				panic("no bucket was found for given exemplar value")
+				// The +Inf bucket should be explicitly added if there is an exemplar for it, similar to non-const histogram logic in https://github.com/prometheus/client_golang/blob/main/prometheus/histogram.go#L357-L365.
+				b := &dto.Bucket{
+					CumulativeCount: proto.Uint64(pb.Histogram.Bucket[len(pb.Histogram.GetBucket())-1].GetCumulativeCount()),
+					UpperBound:      proto.Float64(math.Inf(1)),
+					Exemplar:        e,
+				}
+				pb.Histogram.Bucket = append(pb.Histogram.Bucket, b)
+				break
+				// Terminating the loop after creating the +Inf bucket and adding one exemplar, if there are other exemplars in the +Inf bucket range they will be ignored.
 			}
 		}
 	default:

prometheus/metric_test.go

@@ -14,6 +14,7 @@
 package prometheus
 
 import (
+	"math"
 	"testing"
 
 	//nolint:staticcheck // Ignore SA1019. Need to keep deprecated package for compatibility.
@@ -56,16 +57,19 @@ func TestWithExemplarsMetric(t *testing.T) {
 			{Value: proto.Float64(89.0)},
 			{Value: proto.Float64(100.0)},
 			{Value: proto.Float64(157.0)},
+			{Value: proto.Float64(500.0)},
+			{Value: proto.Float64(2000.0)},
 		}}
 		metric := dto.Metric{}
 		if err := m.Write(&metric); err != nil {
 			t.Fatal(err)
 		}
-		if want, got := 4, len(metric.GetHistogram().Bucket); want != got {
+		if want, got := 5, len(metric.GetHistogram().Bucket); want != got {
 			t.Errorf("want %v, got %v", want, got)
 		}
 
-		expectedExemplarVals := []float64{24.0, 42.0, 100.0, 157.0}
+		// When there are more exemplars than there are buckets, a +Inf bucket will be created and the last exemplar value will be added.
+		expectedExemplarVals := []float64{24.0, 42.0, 100.0, 157.0, 500.0}
 		for i, b := range metric.GetHistogram().Bucket {
 			if b.Exemplar == nil {
 				t.Errorf("Expected exemplar for bucket %v, got nil", i)
@@ -74,5 +78,11 @@ func TestWithExemplarsMetric(t *testing.T) {
 				t.Errorf("%v: want %v, got %v", i, want, got)
 			}
 		}
+
+		infBucket := metric.GetHistogram().Bucket[len(metric.GetHistogram().Bucket)-1].GetUpperBound()
+
+		if infBucket != math.Inf(1) {
+			t.Errorf("want %v, got %v", math.Inf(1), infBucket)
+		}
 	})
 }