Facility for rate-limited warnings

What shall we do when, as an onion service running in rproxy mode, we try to connect to a local service and get ECONNREFUSED? We don't want to kill off the whole proxy, but we would like to inform the administrator that this has happened. But we don't want to log a warning every time that this warning happens: it could potentially happen every time a user tries to connect, which could be many thousands of times for a busy service. (There are other options besides logging a warning, but they are orthogonal, so let's not talk about them here.)

This is a specific case of a more general issue: Sometimes we want to produce a warning message for a condition that can happen many, many times, but we don't want to spam the logs.

In C tor, we have a facility to produce rate-limited logs. We use this for messages where we want to say "this is happening", "this is happening a lot" and/or "this is still happening". I propose that we add a similar facility in Arti.

I'm planning to do this unless somebody says it's a bad idea. (@Diziet @gabi-250?)