Add connection stats to MetricsPort documentation

@dgoulet made progress on tpo/core/tor#40194 (closed) and the first things landed on main. We should update the documentation for our MetricsPort feature accordingly.

added Next Q4 labels

assigned to @gk

removed Next label

added Doing label

changed time estimate to 3h

mentioned in merge request !134 (merged)

Alright, I reviewed recent code changes figuring out what we have in the latest stable wrt MetricsPort data and talked to @dgoulet about what we should include, re-arranged things are bit and clarified misleading explanations. I think we are in a better shape now. :)

added 4h of time spent

mentioned in commit 7d0540fb

closed with merge request !134 (merged)