common.proto

syntax = 'proto3';
package common;

message EntityRef {
  string type = 1;
  string id = 2;
}

sample.proto

syntax = 'proto3';
package sample;

import "common.proto";

service Sample {
  rpc GetSample(GetSampleRequest) returns (GetSampleResponse) {}
}

message GetSampleRequest {
  common.EntityRef entity = 1;
}

// Response just replies with the request for demo purposes.
message GetSampleResponse {
  common.EntityRef entity = 1;
}