Additional collator revamp metrics

[tdimitrov]

On top of #12282

Additinal metrics for the collator protocol revamp:

• _validator_advertisements_total - number of received advertisements, by outcome (accepted, rejected + the exact reason as a label)
• _validator_collations_seconded_total - collations sent to backing for secondig
• _validator_slashes_total - collator slashes
• _validator_approved_peer_signals_total - number of ApprovedPeer UMP signals received, grouped by processing outcome (approved, rejected + reason)
• _validator_assigned_paras - how many paras are currently assigned
• _validator_connected_collators - number of currently connected collators, grouped by score

Regarding _validator_advertisements_total

All different outcomes are tracked so that we can deduce from the metric why advertisements are getting rejected.

Regarding _validator_connected_collators

Collators are grouped in three different bands:

• "0" - zero score, severely slashed or brand new collator.
• "1-99" - low score, These are relatively new collators building up reputation.
• "100-999" - medium score, established collators.
• "1000+" - high score, highly trusted collators.

Reviewers, please pay attention to the following questions:

• Is advertisement tracking an overkill? I was torn between simple accepted/rejected and the variant currently in the PR. I think having separate error cases is a bit better but I'm not sure we'll actually use them for monitoring.
• Validator score - similar question. I was considering counting 0 reputation peers and 1+ reputation peers but went for the 4 bands.

Partially addresses #10402

[tdimitrov]

/cmd prdoc --audience node_dev --bump patch