Provide Protobuf definition for MetadataService for easier cross-language communication

[chickenlj]

Provide Protobuf definition for MetadataService for easier cross-language communication. For example, service discovery between dubbo-go and dubbo-java

[FoghostCn]

Protobuf definition:

syntax = "proto3";
package org.apache.dubbo.metadata;

option go_package = "dubbo.apache.org/dubbo-go/v3/metadata/triple_api";
option java_package = "org.apache.dubbo.metadata";

service MetadataService {
  rpc GetMetadataInfo(Revision) returns (MetadataInfo) {}
}

message Revision {
  string value = 1;
}

message MetadataInfo {
  string app = 1;
  string revision = 2;
  map<string, ServiceInfo> services = 3;
}

message ServiceInfo {
  string name = 1;
  string group = 2;
  string version = 3;
  string protocol = 4;
  int32 port = 5;
  string path = 6;
  map<string, string> params = 7;
}