add support for batch size in metrics service

[ramaraochavali]

Currently Metrics Service sends all metrics in a single batch and that causes server side gRPC Message Size (typical 4MB) to trip causing metrics not to be sent. Even if we increase the size, we end up with a catching game. The proposal is to add batch_size defaults to the current behaviour.

[adisuissa]

cc @jmarantz as metrics_service codeowner.