[FEATURE] - Monitoring added for scheduler

[uded]

I am quite happy with the Monitor [#317] implementation for jobs, really simple to implement Prometheus metrics.

But...

I still need a method to monitor the scheduler. It would be nice to get information about the number of jobs and queued jobs in the scheduler. Currently, such an implementation would require a rather nasty hook to the metrics endpoint or something similar.

IMHO, the best solution would be to have some event listeners or SchedulerMonitor.

[JohnRoesler]

hey @uded

What sort of monitoring would you like to see from the scheduler? I agree, we could have another simple interface for SchedulerMonitor

[uded]

Honestly, I've wrapped up my implementation, added some magic to monitor it, and forgot about it. I would have to resurrect that code to remember where I had issues. Been a while since I last worked on it, like 4 months or so...