When considering that a URL Metric can have an indefinite freshness TTL (#1940), where freshness then depends exclusively on the ETag, I think we should consider increasing the garbage collection time window to be greater than 1 month for od_url_metrics posts that haven't been updated:

Consider sites that get very little traffic. Just because a page doesn't get visited once a month shouldn't result in the URL Metrics being purged. Maybe something like 3 months or 6 months would be more reasonable for the long tail of sites without much traffic.

The garbage collection TTL should probably have a filter to allow it to be customized (or disabled).

Note that if someone deletes a post there is no logic to attempt to delete any corresponding od_url_metrics posts, since there is no connection between the two. I don't think this is necessarily needed.

Docs will also need to be updated here: