[FEAT] Add num_registrations to log and metrics

[blevkivskyi-everstake]

I have two muxes with different relays, and it would be good to have num_registrations for each of them in the log, for example.

From this:

{"timestamp":"2024-12-23T14:33:36.328688Z","level":"DEBUG","fields":{"message":"registration successful","code":"200","latency":"296.08105ms"},"target":"cb_pbs::mev_boost::register_validator","spans":[{"req_id":"353bdfc2-5a0f-46a6-8027-6b03d632a93d","name":"register_validators"},{"relay_id":"boost-relay-holesky.flashbots.net","retry":0,"name":"handler"}]}

{"timestamp":"2024-12-23T14:33:36.438698Z","level":"DEBUG","fields":{"message":"registration successful","code":"200","latency":"406.05268ms"},"target":"cb_pbs::mev_boost::register_validator","spans":[{"req_id":"353bdfc2-5a0f-46a6-8027-6b03d632a93d","name":"register_validators"},{"relay_id":"bloxroute.holesky.blxrbdn.com","retry":0,"name":"handler"}]}

To this:

{"timestamp":"2024-12-23T14:33:36.328688Z","level":"DEBUG","fields":{"message":"registration successful","code":"200","latency":"296.08105ms", "num_registrations":3},"target":"cb_pbs::mev_boost::register_validator","spans":[{"req_id":"353bdfc2-5a0f-46a6-8027-6b03d632a93d","name":"register_validators"},{"relay_id":"boost-relay-holesky.flashbots.net","retry":0,"name":"handler"}]}

{"timestamp":"2024-12-23T14:33:36.438698Z","level":"DEBUG","fields":{"message":"registration successful","code":"200","latency":"406.05268ms", "num_registrations":3},"target":"cb_pbs::mev_boost::register_validator","spans":[{"req_id":"353bdfc2-5a0f-46a6-8027-6b03d632a93d","name":"register_validators"},{"relay_id":"bloxroute.holesky.blxrbdn.com","retry":0,"name":"handler"}]}