Outbox: Log progress

[obenland]

Logs progress of Outbox processing and makes it accessible through the ap_outbox post type API endpoint to make it easier to understand its success.

Proposed changes:

• Adds a callback that logs Outbox processing progress.

Testing instructions:

• Update a post.
• Make sure Outbox item gets processed.
• Query /wp/v2/ap_outbox and look for the meta key or check the Outbox UI in Debug mode.

[obenland]

@mattwiebe @pfefferle Do you think this is worth adding? Helpful?

[pfefferle]

should we move that to the debug class?

[pfefferle]

I think the overall question is, how do we provide the user a way to either debug the outbox or to provide us relevant data.

Two ideas:

1. So maybe we need to think about a log-view that is visible for the admin, that show exactly that data and maybe allows to manually retry.
2. A report, that might be generated by the "Site Health", that might signal if an outbox does not properly federate any more and maybe some stats that a user can look at under: /wp-admin/site-health.php?tab=debug

This is not an argument against storing the data, but maybe it helps us understand what we want to store and where!?!