Add bench test for reader collect methods (#2922)

sdk/metric/manual_reader_test.go

@@ -26,3 +26,7 @@ import (
 func TestManualReader(t *testing.T) {
 	suite.Run(t, &readerTestSuite{Factory: NewManualReader})
 }
+
+func BenchmarkManualReader(b *testing.B) {
+	b.Run("Collect", benchReaderCollectFunc(NewManualReader()))
+}

sdk/metric/periodic_reader_test.go

@@ -178,3 +178,9 @@ func TestPeriodicReaderRun(t *testing.T) {
 	// Ensure Reader is allowed clean up attempt.
 	_ = r.Shutdown(context.Background())
 }
+
+func BenchmarkPeriodicReader(b *testing.B) {
+	b.Run("Collect", benchReaderCollectFunc(
+		NewPeriodicReader(new(fnExporter)),
+	))
+}

sdk/metric/reader_test.go

@@ -20,6 +20,7 @@ package metric // import "go.opentelemetry.io/otel/sdk/metric/reader"
 import (
 	"context"
 	"sync"
+	"testing"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/suite"
@@ -153,3 +154,27 @@ func (p testProducer) produce(ctx context.Context) (export.Metrics, error) {
 	}
 	return testMetrics, nil
 }
+
+func benchReaderCollectFunc(r Reader) func(*testing.B) {
+	ctx := context.Background()
+	r.register(testProducer{})
+
+	// Store bechmark results in a closure to prevent the compiler from
+	// inlining and skipping the function.
+	var (
+		collectedMetrics export.Metrics
+		err              error
+	)
+
+	return func(b *testing.B) {
+		b.ReportAllocs()
+		b.ResetTimer()
+
+		for n := 0; n < b.N; n++ {
+			collectedMetrics, err = r.Collect(ctx)
+			if collectedMetrics != testMetrics || err != nil {
+				b.Errorf("unexpected Collect response: (%#v, %v)", collectedMetrics, err)
+			}
+		}
+	}
+}