Keep track of transport latency

@ejona86 [wrote](https://github.com/grpc/grpc-java/pull/2695#issuecomment-278081929):

> Long-term, I question if the transports should just keep track of their latency. They know the latency initially via `SETTINGS` ack and then later via flow control pinging.

Start with the discussion linked from pull request #2695, then inspect the transport implementations mentioned in the issue. Determine the intended latency data from SETTINGS acknowledgements and flow-control pings, since no files, tests, or concrete consumer are specified. The issue needs an agreed scope and validation criteria before implementation can be considered done.

Scritto dal modello di indicizzazione a partire dal testo della issue.