Document the Reader and Exporter concurrent safe requirements (#4381)

CHANGELOG.md

@@ -19,6 +19,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add info and debug logging to the metric SDK. (#4315)
 - The `go.opentelemetry.io/otel/semconv/v1.21.0` package.
   The package contains semantic conventions from the `v1.21.0` version of the OpenTelemetry Semantic Conventions. (#4362)
+- Document the `Temporality` and `Aggregation` methods of the `"go.opentelemetry.io/otel/sdk/metric".Exporter"` need to be concurrent safe. (#4381)
 
 ### Changed
 

sdk/metric/exporter.go

@@ -30,9 +30,15 @@ var ErrExporterShutdown = fmt.Errorf("exporter is shutdown")
 // the final component in the metric push pipeline.
 type Exporter interface {
 	// Temporality returns the Temporality to use for an instrument kind.
+	//
+	// This method needs to be concurrent safe with itself and all the other
+	// Exporter methods.
 	Temporality(InstrumentKind) metricdata.Temporality
 
 	// Aggregation returns the Aggregation to use for an instrument kind.
+	//
+	// This method needs to be concurrent safe with itself and all the other
+	// Exporter methods.
 	Aggregation(InstrumentKind) aggregation.Aggregation
 
 	// Export serializes and transmits metric data to a receiver.

sdk/metric/reader.go

@@ -65,9 +65,15 @@ type Reader interface {
 	RegisterProducer(Producer)
 
 	// temporality reports the Temporality for the instrument kind provided.
+	//
+	// This method needs to be concurrent safe with itself and all the other
+	// Reader methods.
 	temporality(InstrumentKind) metricdata.Temporality
 
 	// aggregation returns what Aggregation to use for an instrument kind.
+	//
+	// This method needs to be concurrent safe with itself and all the other
+	// Reader methods.
 	aggregation(InstrumentKind) aggregation.Aggregation // nolint:revive  // import-shadow for method scoped by type.
 
 	// Collect gathers and returns all metric data related to the Reader from

sdk/metric/reader_test.go

@@ -168,6 +168,18 @@ func (ts *readerTestSuite) TestMethodConcurrentSafe() {
 	var wg sync.WaitGroup
 	const threads = 2
 	for i := 0; i < threads; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_ = ts.Reader.temporality(InstrumentKindCounter)
+		}()
+
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_ = ts.Reader.aggregation(InstrumentKindCounter)
+		}()
+
 		wg.Add(1)
 		go func() {
 			defer wg.Done()