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

[jcstein]

closing as stale. node team can reopen if they'd like

here's the notion doc i made for this https://www.notion.so/celestiaorg/Celestia-node-metrics-updates-for-documentation-1132a70c9515800f991df7d38e9e0cd6?pvs=4