metrics over libp2p streams

[MarcoPolo]

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Allows a node to expose its metrics to another libp2p node. Only allow listed peer ids can query the metrics.

This enables a user to share their metrics with us, and for us to use our own grafana+prometheus stack to ingest those metrics.

This is more or less done, but it would be helpful for someone else to try setting this up and add to the sparse documentation. Maybe a good task for a motivated person from the PLDG cohort?