feature request: document types of node metrics

[jcstein]

https://github.com/celestiaorg/go-header/blob/main/sync/metrics.go#L39 https://github.com/celestiaorg/go-header/blob/main/store/metrics.go#L26 https://github.com/celestiaorg/go-header/blob/main/p2p/subscriber_metrics.go https://github.com/celestiaorg/go-header/blob/main/p2p/exchange_metrics.go https://github.com/celestiaorg/go-header/blob/main/p2p/server_metrics.go

cc @Wondertan

[Bidon15]

This should be a weekly long task