Add metric to monitor "save code now" efficiency

This only deals with the finalized save code now statuses (succeeded, failed) as it's the only information referenced in the model.

Related to swh/infra/sysadm-environment#1481 (closed)

direct metrics:

which allows to have a rate tendency graph:

avg over time:

Test Plan

docker, save code now and checking metrics out of the endpoint and prometheus graph as in the screenshots ^

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Migrated from D5463 (view on Phabricator)