Have metrics in prometheus for each tracked forge

for each tracked "forge":

• the number of origins that have 0 visits (should decrease)

• the average distance between visits for all origins (ditto): this should be a more generic metric than measuring the "lag" from known updates in the forge, as we won't have that for all forges

and for GitHub and all other forges for which we have push-style notifications of updates:

• the average lag time between when an update happens in the forge and when we start a corresponding ingestion

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Migrated from T1363 (view on Phabricator)