Adding connection limit stats for database and roles

[jocelynthode]

Proposal

Hey, at my company we're relying heavily on the postgres_exporter to monitor our PostgreSQL instances and we recently faced the issues where a project used all their alloted connections defined by a connection limit on the database and we had no way of seeing it in our Prometheus/Alertmanager setup.

I would like to contribute a collector to retrieve both the pg_database_connection_limits and pg_roles_connection_limits. Is this a feature the project would be interested? If yes I'll happily open a PR with these changes. (I already have an initial working code with no tests)

[SuperQ]

Yes, that sounds like perfectly reasonable collectors. Can you document what queries are required for this info?

One thing I'm working towards is consolidating similar collectors.

[jocelynthode]

Here are the required queries:

• As we filter database I have to do it one by one as we do for the database size select pg_database.datconnlimit FROM pg_database WHERE pg_database.datname = $1
• select pg_roles.rolname,pg_roles.rolconnlimit FROM pg_roles

I should have a PR ready either today or by the end of the week. It might be a WIP as I'm not familiar with the code base :)