StreamingContext doesn't change so reuse it

grpc streaming RPC contexts don't change so we
can consider them lifetime contexts and thus we
can record the initial keys once and reuse the
same context when recording measurements.

interceptor/opencensus/opencensus.go

@@ -90,17 +90,17 @@ func (oci *OCInterceptor) Export(tes agenttracepb.TraceService_ExportServer) err
 		return errTraceExportProtocolViolation
 	}
 
-	spansMetricsFn := internal.NewReceivedSpansRecorder("opencensus")
+	spansMetricsFn := internal.NewReceivedSpansRecorderStreaming(tes.Context(), "opencensus")
 
-	processReceivedSpans := func(ctx context.Context, ni *commonpb.Node, spans []*tracepb.Span) {
+	processReceivedSpans := func(ni *commonpb.Node, spans []*tracepb.Span) {
 		// Firstly, we'll add them to the bundler.
 		if len(recv.Spans) > 0 {
 			bundlerPayload := &spansAndNode{node: ni, spans: recv.Spans}
 			traceBundler.Add(bundlerPayload, len(bundlerPayload.spans))
 		}
 
 		// We MUST unconditionally record metrics from this reception.
-		spansMetricsFn(ctx, ni, recv.Spans)
+		spansMetricsFn(ni, recv.Spans)
 	}
 
 	var lastNonNilNode *commonpb.Node
@@ -115,7 +115,7 @@ func (oci *OCInterceptor) Export(tes agenttracepb.TraceService_ExportServer) err
 		// change but we haven't examined all the corner cases for gRPC streaming
 		// hence we'll always pass in the context, in case the underlying implementation
 		// changes a detail. TODO (@odeke-em) investigate if streamer.Context() can change.
-		processReceivedSpans(tes.Context(), lastNonNilNode, recv.Spans)
+		processReceivedSpans(lastNonNilNode, recv.Spans)
 
 		recv, err = tes.Recv()
 		if err != nil {

internal/observability.go

@@ -50,14 +50,18 @@ var AllViews = []*view.View{
 	ViewReceivedSpansInterceptor,
 }
 
-// NewReceivedSpansRecorder creates a function that uses a context created
+// NewReceivedSpansRecorderStreaming creates a function that uses a context created
 // from the name of the interceptor to record the number of the spans received
 // by the interceptor.
-func NewReceivedSpansRecorder(interceptorName string) func(context.Context, *commonpb.Node, []*tracepb.Span) {
-	return func(parentCtx context.Context, ni *commonpb.Node, spans []*tracepb.Span) {
-		ctx, _ := tag.New(parentCtx, tag.Upsert(tagKeyInterceptorName, interceptorName))
-		// TODO: (@odeke-em) perhaps also record information from the node?
+func NewReceivedSpansRecorderStreaming(lifetimeCtx context.Context, interceptorName string) func(*commonpb.Node, []*tracepb.Span) {
+	// We create and reuse this context because for streaming RPCs e.g. with gRPC
+	// the context doesn't change, so it is more useful for avoid expensively adding
+	// keys on each invocation. We can create the context once and then reuse it
+	// when recording measurements.
+	ctx, _ := tag.New(lifetimeCtx, tag.Upsert(tagKeyInterceptorName, interceptorName))
 
+	return func(ni *commonpb.Node, spans []*tracepb.Span) {
+		// TODO: (@odeke-em) perhaps also record information from the node?
 		stats.Record(ctx, mReceivedSpans.M(int64(len(spans))))
 	}
 }