Bug fix: Prevent exemplar drop when filtered attributes exceed 128-rune limit

CHANGELOG.md

@@ -15,6 +15,10 @@ The next release will require at least [Go 1.25].
 
 - Support testing of [Go 1.26]. (#7902)
 
+### Fixed
+
+- Fix dropping entire exemplar when filtered attributes exceed the 128-rune limit.(#7883)
+
 <!-- Released section -->
 <!-- Don't change this section unless doing release -->
 

exporters/prometheus/exporter.go

@@ -12,6 +12,7 @@ import (
 	"slices"
 	"strings"
 	"sync"
+	"unicode/utf8"
 
 	"github.com/prometheus/client_golang/prometheus"
 	dto "github.com/prometheus/client_model/go"
@@ -40,6 +41,13 @@ const (
 	// Registerer, rather than round-tripping them through the bridge and
 	// exporter.
 	bridgeScopeName = "go.opentelemetry.io/contrib/bridges/prometheus"
+
+	exemplarRuneLimit = 128
+)
+
+var (
+	traceKeyLen = utf8.RuneCountInString(otlptranslator.ExemplarTraceIDKey)
+	spanKeyLen  = utf8.RuneCountInString(otlptranslator.ExemplarSpanIDKey)
 )
 
 var metricsPool = sync.Pool{
@@ -304,6 +312,7 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 				addSumMetric(ch, v, m, name, kv, c.labelNamer, c.inst, ctx)
 			case metricdata.Sum[float64]:
 				addSumMetric(ch, v, m, name, kv, c.labelNamer, c.inst, ctx)
+
 			case metricdata.Gauge[int64]:
 				addGaugeMetric(ch, v, m, name, kv, c.labelNamer, c.inst, ctx)
 			case metricdata.Gauge[float64]:
@@ -780,15 +789,25 @@ func addExemplars[N int64 | float64](
 		return m
 	}
 	promExemplars := make([]prometheus.Exemplar, len(exemplars))
+
 	for i, exemplar := range exemplars {
-		labels, err := attributesToLabels(exemplar.FilteredAttributes, labelNamer)
+		traceId := hex.EncodeToString(exemplar.TraceID)
+		spanId := hex.EncodeToString(exemplar.SpanID)
+
+		remaingSpace := exemplarRuneLimit - traceKeyLen - utf8.RuneCountInString(
+			traceId,
+		) - spanKeyLen - utf8.RuneCountInString(
+			spanId,
+		)
+
+		labels, err := attributesToLabels(exemplar.FilteredAttributes, labelNamer, remaingSpace)
 		if err != nil {
 			otel.Handle(err)
 			return m
 		}
 		// Overwrite any existing trace ID or span ID attributes
-		labels[otlptranslator.ExemplarTraceIDKey] = hex.EncodeToString(exemplar.TraceID)
-		labels[otlptranslator.ExemplarSpanIDKey] = hex.EncodeToString(exemplar.SpanID)
+		labels[otlptranslator.ExemplarTraceIDKey] = traceId
+		labels[otlptranslator.ExemplarSpanIDKey] = spanId
 		promExemplars[i] = prometheus.Exemplar{
 			Value:     float64(exemplar.Value),
 			Timestamp: exemplar.Time,
@@ -805,14 +824,26 @@ func addExemplars[N int64 | float64](
 	return metricWithExemplar
 }
 
-func attributesToLabels(attrs []attribute.KeyValue, labelNamer otlptranslator.LabelNamer) (prometheus.Labels, error) {
-	labels := make(map[string]string)
+func attributesToLabels(
+	attrs []attribute.KeyValue,
+	labelNamer otlptranslator.LabelNamer,
+	remainingSpace int,
+) (prometheus.Labels, error) {
+	labels := make(map[string]string, len(attrs)+2)
+	if remainingSpace <= 0 {
+		return labels, nil
+	}
 	for _, attr := range attrs {
 		name, err := labelNamer.Build(string(attr.Key))
 		if err != nil {
 			return nil, err
 		}
-		labels[name] = attr.Value.Emit()
+		val := attr.Value.Emit()
+		attrLength := utf8.RuneCountInString(val) + utf8.RuneCountInString(name)
+		if attrLength <= remainingSpace {
+			labels[name] = val
+			remainingSpace -= attrLength
+		}
 	}
 	return labels, nil
 }

exporters/prometheus/exporter_test.go

@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -720,6 +721,14 @@ func TestPrometheusExporter(t *testing.T) {
 							MaxSize: 10,
 						}},
 					),
+					metric.NewView(
+						metric.Instrument{Name: "test_dropped_attrs_limit"},
+						metric.Stream{
+							AttributeFilter: func(kv attribute.KeyValue) bool {
+								return kv.Key != "long_attribute"
+							},
+						},
+					),
 				),
 			)
 			meter := provider.Meter(
@@ -1291,6 +1300,26 @@ func TestExemplars(t *testing.T) {
 			expectedLabels:        expectedNonEscapedLabels,
 			strategy:              otlptranslator.NoTranslation,
 		},
+		{
+			name: "exemplar overflow does not drop exemplar",
+			recordMetrics: func(ctx context.Context, meter otelmetric.Meter) {
+				hist, err := meter.Float64Histogram("exponential_histogram")
+				require.NoError(t, err)
+
+				// Create attributes that exceed 128-rune limit after accounting for trace_id/span_id
+				// trace_id (32) + span_id (16) + "=" (1) = 49 characters
+				// Remaining space: 128 - 49 = 79 characters
+				// longVal (80 chars) + "=" = 81 chars ✓
+				//  81 chars > 79, so long_value should be truncated
+
+				longVal := strings.Repeat("B", 80) // 80 chars
+
+				hist.Record(ctx, 0, otelmetric.WithAttributes(
+					attribute.String("long_value", longVal),
+				), attrsOpt)
+			},
+			expectedLabels: expectedEscapedLabels,
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			// initialize registry exporter