re-audit and verify prometheus configuration and roadmap matches icinga

in #41712, we found a icinga check that wasn't covered by tpa-rfc-33. this is concerning: i thought I made an exhaustive list of all checks and assumed that our roadmap covered everything.

because we will not be running the full configuration of both monitoring servers in parallel, it's important that we have an exhaustive list of all the checks, and all the metrics they provide.

review all the checks currently configured in icinga and make sure we have a plan to replace them all (or retire them explicitly).

this needs to be done at least a few weeks before icinga is retired (#40695 (closed))