Add metrics to track received / sent bytes over rpc by method

[jimmygchen]

Description

It would be useful if we could track the inbound & outbound bandwidth usage by RPC methods.

Something like:

libp2p_rpc_requests_sent_bytes{type}
libp2p_rpc_response_received_bytes{type}

Note: we have an existing metrics libp2p_rpc_requests_total, but only tracking outgoing request count. Not sure if we have something that track the number of responses sent.