Transport-level bandwidth reporter

[adlrocha]

Adding this here as it was discussed off-band. Right now libp2p isn't capable of giving the amount of total raw data exchanged between two nodes (after the muxer and crypto framings). The only bandwidth measurement available is at a stream level through go-libp2p-core/metrics/bandwidth.go: TotalBandwidth(). A wire-level measurement could come pretty handy, especially when debugging implementations after the muxer.

@raulk: To achieve this we may want to implement a transport wrapper that wraps connections and reports physical bytes read and written to the Reporter. For this we’d need to enhance the Reporter interface (or add a secondary Reporter interface that we can type assert against to know if the implementation has that feature).
Thus we will have two levels of bandwidth measurements in libp2p:

• The transport would tell us how many bytes were written/read off the wire (disregarding the L3/L4 protocol framing), per connection (which we can then map to the peer).
• The higher-up component can tell us how many bytes were written/read per protocol and per stream.
  We probably want a single sink for these stats, with various reporting sources.

cc @aarshkshah1992

[marten-seemann]

What exactly would count in the case of QUIC? Only STREAM frames, all frames, or all packets? And how should retransmissions be handled?