Option to close channel or suppress ResourceWarning

[redramen]

Hey folks, seems like the python mlmd library lets it's grpc channel get autoclosed resulting in a ResourceWarning log message being printed out after a while. Would it be possible to expose the underlying grpc channel in the stub/client so users can manually close the channel or at least suppressing the resource warning log?

[hughmiao]

@redramen , what is the release version that you are using? Would you please provide an output log too?