The cron and the prometheus.metric table only existed to pass values from
the cron process to the HTTP workers. Querying the database directly when
/metrics is scraped removes the cron, the table and the staleness.
This commit features a refactoring of monitoring_prometheus to get a
cron gathering some metrics from the database, and then publishing the
result on the /metrics endpoint
The new module monitoring_prometheus_queue_job extends the cron to get
information about the queue job states.
David Gallay
Alexandre Fayolle
Patrick Tombez