Add TestFixedSizeExemplarConcurrentSafe

This verifies this as a fix to #5814.

sdk/metric/exemplar_test.go

@@ -0,0 +1,62 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package metric // import "go.opentelemetry.io/otel/sdk/metric"
+
+import (
+	"context"
+	"runtime"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+)
+
+func TestFixedSizeExemplarConcurrentSafe(t *testing.T) {
+	// Tests https://github.com/open-telemetry/opentelemetry-go/issues/5814
+
+	t.Setenv("OTEL_METRICS_EXEMPLAR_FILTER", "always_on")
+
+	r := NewManualReader()
+	m := NewMeterProvider(WithReader(r)).Meter("exemplar-concurrency")
+	// Use two instruments to get concurrent access to any shared globals.
+	i0, err := m.Int64Counter("counter.0")
+	require.NoError(t, err)
+	i1, err := m.Int64Counter("counter.1")
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	add := func() {
+		i0.Add(ctx, 1)
+		i1.Add(ctx, 2)
+	}
+
+	goRoutines := max(10, runtime.NumCPU())
+
+	var wg sync.WaitGroup
+	for n := 0; n < goRoutines; n++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for {
+				select {
+				case <-ctx.Done():
+					return
+				default:
+					require.NotPanics(t, add)
+				}
+			}
+		}()
+	}
+
+	const collections = 100
+	var rm metricdata.ResourceMetrics
+	for c := 0; c < collections; c++ {
+		require.NotPanics(t, func() { _ = r.Collect(ctx, &rm) })
+	}
+
+	cancel()
+	wg.Wait()
+}