A gauge is more useful as we can visualise it and combine it with
other stats - we already have other stats for the total number of
notifications sent by provider, and we can extrapolate the number
of slow notifications using this, if needed.
We also still have logs to say the task is running, as well as a
log in the calling code when we actually make a switch [1], so
we're not losing anything by removing the log here.
[1]: a9306c4557/app/celery/scheduled_tasks.py (L117)
11278c47f5