p2p: Move user_message metrics counters to p2p_conn

The following discussion from !7637 (merged) should be addressed:

• @Saroupille started a discussion: (+3 comments)

  This metrics (but also the corresponding metric for the received case) seems a bit fragile. The invariant that the counter is always increased when a message is sent could be broken if the API exposes a new function and does not increase the counter.

  Maybe it would be better to have a single place to increment such a counter. I understand that from the API and the way the code is written, it is not obvious though.