Adds the following peer-to-peer metrics:

- Peer count
- Stream count
- Number of gossip events by type
- Peer-to-peer bandwidth usage by direction

I used libp2p's existing interfaces wherever I could. Note that we still need a backgorund goroutine to pull banwdiwth stats out of the libp2p bandwidth counter.

Meta:

- Fixes ENG-2338
- Fixes ENG-2336
- Fixes ENG-2329