An implementation that serializes metrics to a protocol buffer.
This is a merge from wvgerrit/28440.
I intend to submit 2048751, 2048750, and 2048509 together.
Bug: 36217927
Bug: 36220975
Test: Added unit tests to cover modified code.
Change-Id: Ie8b9d8b91d2602b015f5568890a16c0419c126df