stats/opencensus: Fix flaky metrics test (#6372)

Author: zasweq

stats/opencensus/e2e_test.go

@@ -237,6 +237,36 @@ func distributionDataLatencyCount(vi *viewInformation, countWant int64, wantTags
 	return nil
 }
 
+// waitForServerCompletedRPCs waits until both Unary and Streaming metric rows
+// appear, in two separate rows, for server completed RPC's view. Returns an
+// error if the Unary and Streaming metric are not found within the passed
+// context's timeout.
+func waitForServerCompletedRPCs(ctx context.Context) error {
+	for ; ctx.Err() == nil; <-time.After(time.Millisecond) {
+		rows, err := view.RetrieveData("grpc.io/server/completed_rpcs")
+		if err != nil {
+			continue
+		}
+		unaryFound := false
+		streamingFound := false
+		for _, row := range rows {
+			for _, tag := range row.Tags {
+				if tag.Value == "grpc.testing.TestService/UnaryCall" {
+					unaryFound = true
+					break
+				} else if tag.Value == "grpc.testing.TestService/FullDuplexCall" {
+					streamingFound = true
+					break
+				}
+			}
+			if unaryFound && streamingFound {
+				return nil
+			}
+		}
+	}
+	return fmt.Errorf("timeout when waiting for Unary and Streaming rows to be present for \"grpc.io/server/completed_rpcs\"")
+}
+
 // TestAllMetricsOneFunction tests emitted metrics from gRPC. It registers all
 // the metrics provided by this package. It then configures a system with a gRPC
 // Client and gRPC server with the OpenCensus Dial and Server Option configured,
@@ -987,10 +1017,13 @@ func (s) TestAllMetricsOneFunction(t *testing.T) {
 			},
 		},
 	}
-	// Unregister all the views. Unregistering a view causes a synchronous
-	// upload of any collected data for the view to any registered exporters.
-	// Thus, after this unregister call, the exporter has the data to make
-	// assertions on immediately.
+	// Server Side stats.End call happens asynchronously for both Unary and
+	// Streaming calls with respect to the RPC returning client side. Thus, add
+	// a sync point at the global view package level for these two rows to be
+	// recorded, which will be synchronously uploaded to exporters right after.
+	if err := waitForServerCompletedRPCs(ctx); err != nil {
+		t.Fatal(err)
+	}
 	view.Unregister(allViews...)
 	// Assert the expected emissions for each metric match the expected
 	// emissions.