WIP: gossipsub: add optional per-peer delay metrics

[cskiraly]

This is to add more delay related metrics to evaluate GossipSub improvement proposals.

See https://github.com/libp2p/specs/pull/413 for related discussions

[cskiraly]

The current version is one that exposes lots of data: one histogram per peer. Thus, the metric is kept optional. Later we might streamline this to collect only stats like mean and maybe stddev.

I'm also wondering whether we should add this here, or move it to scoring.nim.

It would also be nice to collect some delay related data on first arrivals, for example delay between the first and the second arrival.

[Menduist]

Just a note, we won't be able to enable per-peer metric on the fleet (too expensive) We can still get this in though