Add task related metrics to swh-scheduler

Add some task related metrics to the swh-scheduler base task class (or whatever replaces it):

• task counts (start, success, failure)
• task durations (histogram)

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Migrated from T1460 (view on Phabricator)

mentioned in commit c29d3834

marked this issue as related to swh/meta#1408 (closed)

added Metrics/monitoring Scheduling utilities priority:High labels

assigned to @olasd

closed

With statsd-exporter deployed, we have a dashboard:

https://grafana.softwareheritage.org/d/b_xh3f9ik/worker-task-processing

The dashboard can be configured to display metrics about a specific task type by selecting it in the dropdown:

https://grafana.softwareheritage.org/d/b_xh3f9ik/worker-task-processing?var-task=swh.loader.git.tasks.UpdateGitRepository&orgId=1

mentioned in merge request !47 (closed)