to analyze: add new metrics

[richard-ramos]

In #586 @chaitanyaprem suggested the following improvements:

• Add count of peerExchange requests handled as well.
• Include a metric to show number of peers in peerstore
• Disclose in and out connected peer count separately (going as a label to the connectedPeerCount?)
• Consider maintaining discoveredNode metrics via separate counter per protocol or have a single one which gets updated with label as protocolName.
• Duplicated proof (see chore(rln-relay): clean up nullifier table every MaxEpochGap #715 (comment))

cc: @jm-clius @alrevuelta as these metrics might make sense to be added in nwaku as well (they might even exist already, but i have not checked)

[chaitanyaprem]

Closing the issue as it is descoped for go-waku.