Add counter for unapproved candidates (#7491)

* Add counter for unapproved candidates

* Update metrics

* Split metrics

* Remove depth metric

* Print only the oldest unapproved candidates

* Update logging condition

* Fix logging condition

* Update logging

* Update node/core/approval-voting/src/lib.rs

Co-authored-by: Andrei Sandu <[email protected]>

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Co-authored-by: Andrei Sandu <[email protected]>