swarm/: Expose time to establish connection

[mxinden]

Description

Expose the time it takes to establish an inbound or outbound connection.

Motivation

Needed for monitoring purposes, e.g. to be exposed to Prometheus via libp2p-metrics. Especially relevant now that we are adding two more transport protocols and thus might want to compare them. #2622 #2289

Open questions

• Likely best tracked in Pool, thus not requiring Transport to be aware of it.

Are you planning to do it yourself in a pull request?

Likely not.