Expose jobrunner metrics

[sbidoul]

This is a first draft towards #965

This turns out to be fairly simple. A few things worth noting:

• The http server runs in the same thread and event loop as listen/notify job events, so there are no concurrency issues while accessing the channels stats
• I added the set of jobs waiting for dependencies on each channel for completeness and convenient access to that stat, similar to what is done for failed jobs.
• I used the prometheus-client external library. It's currently required even if the metrics server is not configured. This could be improved by mocking the lib and making it mandatory only for users enabling the metrics server. Or make it a required dependency (which is my preference, for simplicity).
• This still needs tests and docs.

Test by setting ODOO_QUEUE_JOB_METRICS_PORT=8080 before running Odoo then watch curl -s http://localhost:8080.

Assisted-By: Pamina (my cat)

[OCA-git-bot]

Hi @guewen,
some modules you are maintaining are being modified, check this out!

[sbidoul]

Maybe move this to configure? Because the leaf label is probably not correct if/when children are added after the parent.