p2p: move metrics closer to the measured action

What

Counting the number of message sent was done from P2p instead of P2p_conn module because when using P2p_metrics in P2p_conn, there was a cyclic dependency. !8809 (merged) split P2p_metrics and reduced module dependencies. Then, now it is possible to move these metrics to a better place, that may prevent to forget to count sent messages in future modifications.

Why

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR