Pass streaming context when recording spans

Addressing feedback from review.

* Added a TODO to investigate
if/when a streamer's Context can change
* Fixed unnecessary capitalization

interceptor/opencensus/opencensus.go

@@ -90,17 +90,17 @@ func (oci *OCInterceptor) Export(tes agenttracepb.TraceService_ExportServer) err
 		return errTraceExportProtocolViolation
 	}
 
-	spansMetricsFn := internal.NewReceivedSpansRecorder(context.Background(), "opencensus")
+	spansMetricsFn := internal.NewReceivedSpansRecorder("opencensus")
 
-	processReceivedSpans := func(ni *commonpb.Node, spans []*tracepb.Span) {
+	processReceivedSpans := func(ctx context.Context, 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(ni, recv.Spans)
+		// We MUST unconditionally record metrics from this reception.
+		spansMetricsFn(ctx, ni, recv.Spans)
 	}
 
 	var lastNonNilNode *commonpb.Node
@@ -111,7 +111,11 @@ func (oci *OCInterceptor) Export(tes agenttracepb.TraceService_ExportServer) err
 			lastNonNilNode = recv.Node
 		}
 
-		processReceivedSpans(lastNonNilNode, recv.Spans)
+		// Note: In normal operations, the context of a streaming RPC usually doesn't
+		// 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)
 
 		recv, err = tes.Recv()
 		if err != nil {

internal/observability.go

@@ -50,11 +50,11 @@ var AllViews = []*view.View{
 // NewReceivedSpansRecorder 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(parentCtx context.Context, interceptorName string) func(*commonpb.Node, []*tracepb.Span) {
-	ctx, _ := tag.New(parentCtx, tag.Upsert(tagKeyInterceptorName, interceptorName))
-	// TODO: (@odeke-em) perhaps also record information from the node?
+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?
 
-	return func(ni *commonpb.Node, spans []*tracepb.Span) {
 		stats.Record(ctx, mReceivedSpans.M(int64(len(spans))))
 	}
 }