[http2] Stats and logging for H2 goaway error codes

It would be good to have stat counters for H2 goaway error codes so we can better understand reasons why H2 connections are forced closed by the local or remote endpoint. Requests that failed due to immediate local or remote goaway should goaway error code and debug data in their request logs. It would also be nice to have a config option to enable sending additional debug data in goaway messages to aid debugging.

After doing some code inspection, I think that most of the interesting goaway reasons like flow control errors and compression problems are generated inside the nghttp2 library and there may not be an existing debug hook that we could hook into for stats.