fix prometheus interceptors not converting context errors to gRPC cod…

…es (grpc-ecosystem#571)

- Previously context.(Canceled|DeadlineExceeded) would result in an
  "Unknown" grpc_code

providers/prometheus/options.go

@@ -9,11 +9,15 @@ import (
 	"google.golang.org/grpc/status"
 )
 
-// FromError returns a grpc status if error code is a valid grpc status.
-func FromError(err error) (s *status.Status, ok bool) {
-	return status.FromError(err)
-
-	// TODO: @yashrsharma44 - discuss if we require more error handling from the previous package
+// FromError returns a grpc status. If the error code is neither a valid grpc status nor a context error, codes.Unknown
+// will be set.
+func FromError(err error) *status.Status {
+	s, ok := status.FromError(err)
+	// Mirror what the grpc server itself does, i.e. also convert context errors to status
+	if !ok {
+		s = status.FromContextError(err)
+	}
+	return s
 }
 
 // A CounterOption lets you add options to Counter metrics using With* funcs.

providers/prometheus/reporter.go

@@ -22,7 +22,7 @@ type reporter struct {
 
 func (r *reporter) PostCall(err error, rpcDuration time.Duration) {
 	// get status code from error
-	status, _ := FromError(err)
+	status := FromError(err)
 	code := status.Code()
 
 	// perform handling of metrics from code

providers/prometheus/server_test.go

@@ -143,6 +143,18 @@ func (s *ServerInterceptorTestSuite) TestStreamingIncrementsMetrics() {
 		s.serverMetrics.serverHandledHistogram.WithLabelValues("server_stream", testpb.TestServiceFullName, "PingList"))
 }
 
+func (s *ServerInterceptorTestSuite) TestContextCancelledTreatedAsStatus() {
+	ctx, cancel := context.WithCancel(context.TODO())
+	defer cancel()
+
+	stream, _ := s.Client.PingStream(ctx)
+	stream.Send(&testpb.PingStreamRequest{})
+	cancel()
+
+	requireValueWithRetry(s.SimpleCtx(), s.T(), 1,
+		s.serverMetrics.serverHandledCounter.WithLabelValues("bidi_stream", testpb.TestServiceFullName, "PingStream", "Canceled"))
+}
+
 // fetchPrometheusLines does mocked HTTP GET request against real prometheus handler to get the same view that Prometheus
 // would have while scraping this endpoint.
 // Order of matching label vales does not matter.