api: add static router and improve path parser in rpc handler (#1437)

* api: add static router and improve path parser in rpc handler

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* expose metadata context key to be able to get unmodified map keys

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* server/grpc: fix jsonpb codec for protobuf msg

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* api/handler/rpc: write 204 status code when rsp is nil

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

* api/handler/rpc: add check for nil response for non javascript

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

api/service/proto/api.proto

@@ -0,0 +1,18 @@
+syntax = "proto3";
+
+package go.micro.api;
+
+service Api {
+  rpc Register(Endpoint) returns (EmptyResponse) {};
+  rpc Deregister(Endpoint) returns (EmptyResponse) {};
+}
+
+message Endpoint {
+  string name = 1;
+  repeated string host = 2;
+  repeated string path = 3;
+  repeated string method = 4;
+  bool stream = 5;
+}
+
+message EmptyResponse {}