Merge pull request #3150 from nathanielmanistaatgoogle/metadata-and-serialization

src/python/grpcio/grpc/_links/invocation.py

src/python/grpcio/grpc/_links/service.py

src/python/grpcio/grpc/beta/_stub.py

src/python/grpcio/grpc/beta/beta.py

...io_test/grpc_test/_core_over_links_base_interface_test.py

...c_test/_crust_over_core_over_links_face_interface_test.py

...cio_test/grpc_test/_links/_lonely_invocation_link_test.py

...python/grpcio_test/grpc_test/_links/_transmission_test.py