add performance metrics to gossipsub

[divagant-martian]

Add two metrics that help analyze if certain configurations/network are performing as expected.

[AgeManning]

I think this is ready for review and another handy one to get down before some more gossipsub updates.

Will give a basis for future metrics should we need them.

[mxinden]

🚀

As a side note, I have created a Grafana dashboard for metrics exposed for libp2p-identify, libp2p-ping and libp2p-kad. Maybe we can join efforts and include the Gossipsub metrics as well.

[mxinden]

Mind resolving the merge conflicts now that #2327 is merged? Other than that, this is good to go from my side.

[AgeManning]

Yep. I've got a bunch related to the new metrics (some are lighthouse-specific) but we should be able to rip out the gossipsub ones and combine them.

Do you have a repo somewhere for your dashboards. My current one is here: https://github.com/sigp/lighthouse-metrics/blob/network/dashboards/LighthouseNetwork.json