job manager: collect and return stats

[soxofaan]

MultiBackendJobManager.run_jobs doesn't return anything at the moment.

I'm thinking to let it return a dictionary with stats (e.g. number of sleeps, polls, jobs started, ...) which should be informing to end user, and also should make it easier to implements job manager related tests (avoids having to mock various things, just to get insights)

[soxofaan]

Under PR #644 I already started with returning event stats from MultiBackendJobManager.run_jobs

threaded run_jobs is not covered there yet

[soxofaan]

I just merged #644 so there are now stats from MultiBackendJobManager.run_jobs

next steps:

• also support the threaded variant in some way?
• also support stats when run_jobs fails (and does not return), e.g. by allowing to inject a stats dictionary in run_jobs
• check these current weird stats:

  openeo-python-client/tests/extra/test_job_management.py

  Line 121 in dcc7119

  "start_job call": 7, # TODO?

  openeo-python-client/tests/extra/test_job_management.py

  Line 165 in dcc7119

  "start_job call": 7, # TODO?

  and related

[soxofaan]

also support the threaded variant in some way?

with aa3e653 I already added logging of the stats in threaded mode

check these current weird stats

was fixed as side-effect of e419e14