[BUG]definition service lowercase generation cannot be exported

[songzhibin97]

What happened:

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc sayHello (HelloRequest) returns (HelloReply)  {
    option (google.api.http) = {
        // 定义一个 GET 接口，并且把 name 映射到 HelloRequest
        get: "/helloworld/{name}",
        // 可以添加附加接口
        additional_bindings {
            // 定义一个 POST 接口，并且把 body 映射到 HelloRequest
            post: "/v1/greeter/say_hello",
            body: "*",
        }
    };
  }
}

kratos  proto server

// Unable to export
func (s *GreeterService) sayHello(ctx context.Context, req *pb.HelloRequest) (*pb.HelloReply, error) {
	return &pb.HelloReply{}, nil
}