Improve performance of MonitoringInterceptors (#18)

* Improve performance of MonitoringInterceptors

The current implementation has multiple GrpcMethod.of() calls which are unnecessary. The method in turn calls three String methods (.substring twice, .lastIndexOf once) which add unnecessary overhead which should be avoided when collecting metrics.

* Fix typo

src/main/java/me/dinowernli/grpc/prometheus/ClientMetrics.java

@@ -130,10 +130,10 @@ static class Factory {
       }
     }
 
-    /** Creates a {@link ClientMetrics} for the supplied method. */
-    <R, S> ClientMetrics createMetricsForMethod(MethodDescriptor<R, S> methodDescriptor) {
+    /** Creates a {@link ClientMetrics} for the supplied gRPC method. */
+    ClientMetrics createMetricsForMethod(GrpcMethod grpcMethod) {
       return new ClientMetrics(
-          GrpcMethod.of(methodDescriptor),
+          grpcMethod,
           rpcStarted,
           rpcCompleted,
           streamMessagesReceived,

src/main/java/me/dinowernli/grpc/prometheus/MonitoringClientInterceptor.java

@@ -33,11 +33,12 @@ private MonitoringClientInterceptor(
   @Override
   public <R, S> ClientCall<R, S> interceptCall(
       MethodDescriptor<R, S> methodDescriptor, CallOptions callOptions, Channel channel) {
-    ClientMetrics metrics = clientMetricsFactory.createMetricsForMethod(methodDescriptor);
+    GrpcMethod grpcMethod = GrpcMethod.of(methodDescriptor);
+    ClientMetrics metrics = clientMetricsFactory.createMetricsForMethod(grpcMethod);
     return new MonitoringClientCall<>(
         channel.newCall(methodDescriptor, callOptions),
         metrics,
-        GrpcMethod.of(methodDescriptor),
+        grpcMethod,
         configuration,
         clock);
   }

src/main/java/me/dinowernli/grpc/prometheus/MonitoringServerInterceptor.java

@@ -33,12 +33,12 @@ public <R, S> ServerCall.Listener<R> interceptCall(
       ServerCall<R, S> call,
       Metadata requestHeaders,
       ServerCallHandler<R, S> next) {
-    MethodDescriptor<R, S> method = call.getMethodDescriptor();
-    ServerMetrics metrics = serverMetricsFactory.createMetricsForMethod(method);
-    GrpcMethod grpcMethod = GrpcMethod.of(method);
+    MethodDescriptor<R, S> methodDescriptor = call.getMethodDescriptor();
+    GrpcMethod grpcMethod = GrpcMethod.of(methodDescriptor);
+    ServerMetrics metrics = serverMetricsFactory.createMetricsForMethod(grpcMethod);
     ServerCall<R,S> monitoringCall = new MonitoringServerCall(call, clock, grpcMethod, metrics, configuration);
     return new MonitoringServerCallListener<>(
-        next.startCall(monitoringCall, requestHeaders), metrics, GrpcMethod.of(method));
+        next.startCall(monitoringCall, requestHeaders), metrics, grpcMethod);
   }
 
 }

src/main/java/me/dinowernli/grpc/prometheus/ServerMetrics.java

@@ -134,10 +134,10 @@ static class Factory {
       }
     }
 
-    /** Creates a {@link ServerMetrics} for the supplied method. */
-    <R, S> ServerMetrics createMetricsForMethod(MethodDescriptor<R, S> methodDescriptor) {
+    /** Creates a {@link ServerMetrics} for the supplied gRPC method. */
+    ServerMetrics createMetricsForMethod(GrpcMethod grpcMethod) {
       return new ServerMetrics(
-          GrpcMethod.of(methodDescriptor),
+          grpcMethod,
           serverStarted,
           serverHandled,
           serverStreamMessagesReceived,