logging: configure default logging config

[oliver-sanders]

Provide a default logging setup for the UIS:

• Bump Traitlets to >=5.2.1 (needed for logging_config) trait.
• Add default logging config to write to ~/.cylc/uiserver/uiserver.log at the INFO level.
• Document in the readme for now.

Note this only captures the CylcUIServer part of the log, the base ServerApp log (i.e. the jupyter_server log) and any extensions (e.g. jupyter_lab) are excluded (can be easily added if desired but not much use from the Cylc perspective).

In combination with #348 this should capture workflow state transitions and tracebacks which are useful for debugging and monitoring for issues.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Does not need tests (please test manually)
• Appropriate change log entry included.
• No documentation update required (waiting on feature to be documented in jupyter_server)
• No dependency changes.

[oliver-sanders]

(rebased)

[oliver-sanders]

@wxtim pointed out that uncaptured tracebacks go to the ServerApp log rather than the CylcUIServer log so I've added logging for the ServerApp too (pointing at a different file).

Ignore that, Tim found that an exception raised by the "play" service (within the bit which actually performs the play) appears to get printed rather than logged. As it happens that method makes extensive use of try/except and logs any exceptions which occur so this should not be an issue. I'm not sure why an exception raised there gets printed rather than logged, my guess is that it's related to the ThreadPoolExecutor, perhaps exception handling would need to be configured there. Not a big issue though, we should be try/excepting everything in these methods.

[oliver-sanders]

Copied from Cylc, perhaps we should reduce this a little?

[wxtim]

Given the amount of logging the UIS doesn't produce I feel that this is likely to be overkill.

[oliver-sanders]

I reckon 100'000 bytes should be ~2000 lines worth of log file which seems plenty long enough.

Suggested change

	'maxBytes': 10485760,
	'maxBytes': 100000,

[oliver-sanders]

Could potentially bump this to the 1.0.2 milestone (only adds logging) but no pressure from our side (happy to use dev builds for the moment).

[oliver-sanders]

Rebased, added changelog.

[wxtim]

• Code looks sane
• Working as advertised