Document the Reader and Exporter concurrent safe requirements

The aggregation and temporality methods may be called concurrently. Both
with themselves and with other methods of the Reader or Exporter.
Implementations need to provide concurrent safe methods. This change
adds documentation about this requirement.

Part of open-telemetry#3925

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. (#TBD)
 
 ### 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()