Implement garbage collection for URL metrics posts

[westonruter]

Feature Description

In the Image Loading Optimization module (see #869), detection logic may run when a visitor lands on a page to collect metrics for that URL, namely which element is LCP and which images were in the initial viewport. This information is then sent to a REST API endpoint and is then stored in an ilo_url_metrics post. Over time, URL metrics will be updated in this post as more visitors land on the page. However, a post gets deleted and the URL for which an ilo_url_metrics post was created, then at present the ilo_url_metrics will persist indefinitely. There should be a cron job that does garbage collection of these posts ilo_url_metrics when they are excessively stale. For example, if the post_modified_gmt for a given post is over a month old, then the post could be deleted.

🆕 This should also involve implementing an uninstall.php which deletes all instances of the custom post type at uninstallation.

[thelovekesh]

For example, if the post_modified_gmt for a given post is over a month old, then the post could be deleted.

I think it would be better to let the user set the TTL for an entry. By default, we can keep it to ~30-45 days?

[westonruter]

I don't think we need to let the user set the TTL. The URL metrics post will be continually refreshed according to how long a URL metric is considered fresh. So a URL metrics post will only be garbage-collected if there are zero visits that keep the URL metrics over the course of a month. We can revisit later if this needs to be further configurable. For now let's just delete any posts that haven't been modified in a month.