Akka.IO: enable optional metrics subscriptions for `TcpListener` / `TcpConnection` actors

[Aaronontheweb]

Is your feature request related to a problem? Please describe.

Same idea as #7427, but for Akka.IO actors. We could do it both for UDP and TCP - but the basic idea is the same:

1. Create a TcpListener actor;
2. Send it a Subscribe message of some kind;
3. TcpListener will periodically send us a small struct with a bunch of metrics - they'll be gauge-like (totals, not rates of change).

[Aaronontheweb]

Opted not to do metrics for individual connections - just the TcpListener for now