[Proposal] Track connection pool metrics via jmx

[Daenyth]

I think a blocker for using Skunk in production is the ability to track hikari-equivalent metrics.

Specifically, hikari exposes these

• Idle Connection count
• Active Connections (in use)
• Total Connections
• The number of threads waiting for a connection

They may not exactly translate to the nonblocking api (especially the 'number of threads waiting', but getting something near parity would be massively helpful.

As a bonus, LISTEN-related metrics may be useful too - number of currently active LISTEN streams could be useful to track also.

Some conversation for context: https://gitter.im/skunk-pg/Lobby?at=5f31708aa4768b68568515af