Use sync.OnceValue for metrics registration (#815)

Inspired by a recent Sourcegraph refactor.

cmd/zoekt-sourcegraph-indexserver/main.go

@@ -132,8 +132,21 @@ var (
 		Help: "Counts the number of repos we stopped tracking.",
 	})
 
-	clientMetricsOnce sync.Once
-	clientMetrics     *grpcprom.ClientMetrics
+	// clientMetricsOnce returns a singleton instance of the client metrics
+	// that are shared across all gRPC clients that this process creates.
+	//
+	// This function panics if the metrics cannot be registered with the default
+	// Prometheus registry.
+	clientMetricsOnce = sync.OnceValue(func() *grpcprom.ClientMetrics {
+		clientMetrics := grpcprom.NewClientMetrics(
+			grpcprom.WithClientCounterOptions(),
+			grpcprom.WithClientHandlingTimeHistogram(), // record the overall request latency for a gRPC request
+			grpcprom.WithClientStreamRecvHistogram(),   // record how long it takes for a client to receive a message during a streaming RPC
+			grpcprom.WithClientStreamSendHistogram(),   // record how long it takes for a client to send a message during a streaming RPC
+		)
+		prometheus.DefaultRegisterer.MustRegister(clientMetrics)
+		return clientMetrics
+	})
 )
 
 // 1 MB; match https://sourcegraph.sgdev.org/github.com/sourcegraph/sourcegraph/-/blob/cmd/symbols/internal/symbols/search.go#L22
@@ -1505,7 +1518,7 @@ func internalActorStreamInterceptor() grpc.StreamClientInterceptor {
 const defaultGRPCMessageReceiveSizeBytes = 90 * 1024 * 1024 // 90 MB
 
 func dialGRPCClient(addr string, logger sglog.Logger, additionalOpts ...grpc.DialOption) (proto.ZoektConfigurationServiceClient, error) {
-	metrics := mustGetClientMetrics()
+	metrics := clientMetricsOnce()
 
 	// If the service seems to be unavailable, this
 	// will retry after [1s, 2s, 4s, 8s, 16s] with a jitterFraction of .1
@@ -1559,26 +1572,6 @@ func dialGRPCClient(addr string, logger sglog.Logger, additionalOpts ...grpc.Dia
 	return client, nil
 }
 
-// mustGetClientMetrics returns a singleton instance of the client metrics
-// that are shared across all gRPC clients that this process creates.
-//
-// This function panics if the metrics cannot be registered with the default
-// Prometheus registry.
-func mustGetClientMetrics() *grpcprom.ClientMetrics {
-	clientMetricsOnce.Do(func() {
-		clientMetrics = grpcprom.NewClientMetrics(
-			grpcprom.WithClientCounterOptions(),
-			grpcprom.WithClientHandlingTimeHistogram(), // record the overall request latency for a gRPC request
-			grpcprom.WithClientStreamRecvHistogram(),   // record how long it takes for a client to receive a message during a streaming RPC
-			grpcprom.WithClientStreamSendHistogram(),   // record how long it takes for a client to send a message during a streaming RPC
-		)
-
-		prometheus.DefaultRegisterer.MustRegister(clientMetrics)
-	})
-
-	return clientMetrics
-}
-
 // addDefaultPort adds a default port to a URL if one is not specified.
 //
 // If the URL scheme is "http" and no port is specified, "80" is used.

cmd/zoekt-webserver/main.go

@@ -639,7 +639,7 @@ func traceContext(ctx context.Context) sglog.TraceContext {
 }
 
 func newGRPCServer(logger sglog.Logger, streamer zoekt.Streamer, additionalOpts ...grpc.ServerOption) *grpc.Server {
-	metrics := mustGetServerMetrics()
+	metrics := serverMetricsOnce()
 
 	opts := []grpc.ServerOption{
 		grpc.ChainStreamInterceptor(
@@ -689,24 +689,17 @@ var (
 		Help: "The total number of search requests that zoekt received",
 	})
 
-	serverMetricsOnce sync.Once
-	serverMetrics     *grpcprom.ServerMetrics
-)
-
-// mustGetServerMetrics returns a singleton instance of the server metrics
-// that are shared across all gRPC servers that this process creates.
-//
-// This function panics if the metrics cannot be registered with the default
-// Prometheus registry.
-func mustGetServerMetrics() *grpcprom.ServerMetrics {
-	serverMetricsOnce.Do(func() {
-		serverMetrics = grpcprom.NewServerMetrics(
+	// serviceMetricsOnce returns a singleton instance of the server metrics
+	// that are shared across all gRPC servers that this process creates.
+	//
+	// This function panics if the metrics cannot be registered with the default
+	// Prometheus registry.
+	serverMetricsOnce = sync.OnceValue(func() *grpcprom.ServerMetrics {
+		serverMetrics := grpcprom.NewServerMetrics(
 			grpcprom.WithServerCounterOptions(),
 			grpcprom.WithServerHandlingTimeHistogram(), // record the overall response latency for a gRPC request)
 		)
-
 		prometheus.DefaultRegisterer.MustRegister(serverMetrics)
+		return serverMetrics
 	})
-
-	return serverMetrics
-}
+)