Ray: Clean up and document debug logging for Ray

[pcmoritz]

Currently the logging level "INFO" is hardcoded here:

https://github.com/ray-project/ray/blob/3027dde3037af9924d9ee31533d6c525b30214c6/src/ray/raylet/main.cc#L20

It should be possible to configure that at runtime via ray.init/ray start (e.g. via _internal_config).

[raulchen]

@guoyuhong already implemented this, you can set export RAY_BACKEND_LOG_LEVEL=debug to turn on debugging log.

[pcmoritz]

Ah ok, that's very useful, thanks!

We should consider documenting this in https://ray.readthedocs.io/en/latest/development.html

[stephanie-wang]

Ah, thank you! I'm going to update this issue to add that to the documentation and also clean up some of the debug logging to make it more usable.

[guoyuhong]

Here is the PR: https://github.com/ray-project/ray/pull/3662 .