Add Logging Handler

[waprin]

First stab at addressing #1497.

Starting this with a baby step while we flesh out what we want. So it doesn't worry about fluentd integration, doesn't worry about async transports, doesn't worry about error reporting integration. Just a minimal handler that connects Python logging to Cloud Logging.

[theacodes]

One concern I thought of is thread-safety. Logging is ostensibly thread-safe, but I know for sure our client is not.

[waprin]

For thread safety, I think it's fine because the Python handler base class acquires/releases locks around the emit call.

@tseaver do you think this needs a system test added?

[tseaver]

@waprin a system test would be a good idea. Figuring how how to tear down the handler, as plugged into the stdlib logging module, would be the only hard part, I think.

[waprin]

Yeah I think that's unfortunately not quite right on the unit tests either. I try to undo it but I see log statements in the tox output I usually don't see. I'll look into fixing it and adding a system test.

[waprin]

@tseaver Ok I added system tests and I think logging handlers cleanup is now right in both unit and system tests.

I also added a note to CONTRIBUTING that you need to change service account to Owner for logging. This is required to edit Sinks, not thrilled about it but I'm told it works as intended.

The system tests for logging are all waiting for 2 seconds, they were usually failing for me so I bumped it to 5. Unfortunately logging API is especially eventually-consistent so any of these system tests are always going to be at least a little flaky.

Given there's a bunch more I want to add to improve this handler, as well as more work in this library for Error Reporting, do you think we can start merging all this work into some sort of development branch? I wrote up an internal doc on the game plan, I can share it with you if you like (tseaver at company?). I think the work I have in mind belongs in this repo, but we can discuss, maybe not or maybe you want it as optional dependency or organized into contrib packages.

/cc @filipjs

/cc @steren (no work on error reporting yet, if you'd prefer I can only cc you on PRs relevant to that)

[tseaver]

@waprin I just set up a logging-stdlib-handler feature branch for this work. Can you open a new PR against that branch?

[tseaver]

Superseded by #1920.