Add Handler method to pass in a client (#454)

* Add Handler method to pass in a client

Right now if you want to embed the JSON proxy into your service you end
up having to have a client talking to loopback which is messy. With this
API addition you can simply pass in something which implements the
client interface -- meaning you could wrap it yourself and return
directly (without all the loopback and serialization overhead).

Note that this does not apply interceptors and other gRPC
niceties. Please ensure your application doesn't need
interceptors before using this method.

protoc-gen-grpc-gateway/gengateway/template.go

@@ -338,7 +338,15 @@ func Register{{$svc.GetName}}HandlerFromEndpoint(ctx context.Context, mux *runti
 // Register{{$svc.GetName}}Handler registers the http handlers for service {{$svc.GetName}} to "mux".
 // The handlers forward requests to the grpc endpoint over "conn".
 func Register{{$svc.GetName}}Handler(ctx context.Context, mux *runtime.ServeMux, conn *grpc.ClientConn) error {
-	client := New{{$svc.GetName}}Client(conn)
+	return Register{{$svc.GetName}}HandlerClient(ctx, mux, New{{$svc.GetName}}Client(conn))
+}
+
+// Register{{$svc.GetName}}Handler registers the http handlers for service {{$svc.GetName}} to "mux".
+// The handlers forward requests to the grpc endpoint over the given implementation of "{{$svc.GetName}}Client".
+// Note: the gRPC framework executes interceptors within the gRPC handler. If the passed in "{{$svc.GetName}}Client"
+// doesn't go through the normal gRPC flow (creating a gRPC client etc.) then it will be up to the passed in
+// "{{$svc.GetName}}Client" to call the correct interceptors.
+func Register{{$svc.GetName}}HandlerClient(ctx context.Context, mux *runtime.ServeMux, client {{$svc.GetName}}Client) error {
 	{{range $m := $svc.Methods}}
 	{{range $b := $m.Bindings}}
 	mux.Handle({{$b.HTTPMethod | printf "%q"}}, pattern_{{$svc.GetName}}_{{$m.GetName}}_{{$b.Index}}, func(w http.ResponseWriter, req *http.Request, pathParams map[string]string) {