Instrument a number of DB calls done as part of request and worker

In order to introduce changes as Make Sidekiq to use read-only replicas we need to better understand how many calls we do fire to DB, and whether they are read-only or read-write types.

I think we could add:

• db_readonly_calls: all SELECTs?
• db_readwrite_calls: as they create a sticky connection, all that is not SELECT?
• db_cached_calls: they are still expensive on Rails side to instantiate the objects

And, ideally add it for sidekiq and puma/unicorn. That way we could really see all requests/jobs that perform DB ops, and what kind of ops they do.