client/authority-discovery: Instrument code with Prometheus (#5195)

* client/authority-discovery: Instrument code with Prometheus

Introduce Prometheus metrics into the authority discovery module
enabling one to observe:

- authority_discovery_times_published_total

- authority_discovery_amount_external_addresses_last_published

- authority_discovery_times_requested_total

- authority_discovery_dht_event_received

* client/authority-discovery/src/lib.rs: Reword metric help texts

Co-Authored-By: Ashley <ashley.ruglys@gmail.com>

* client/authority-discovery/src/lib.rs: Reword metric help text

Co-Authored-By: Ashley <ashley.ruglys@gmail.com>

* client/authority-discovery/tests: Fix struct instantiation + basic test

Co-authored-by: Ashley <ashley.ruglys@gmail.com>