Add metrics_labels to MUTABLE_CONFIG_KEYS to introduce the value in 3.13 migrated queues config, in line with 4.2 queues

[elo-magnier-7s]

Proposed Changes

This is the companion to rabbitmq/rabbitmq-server#17184, which makes metrics_labels as an allowed mutable key.

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[michaelklishin]

@elo-magnier-7s I'd like to clarify that we are not adding anything back at the ra level, even if from the system monitoring perspective you might consider that metric "lost".

RabbitMQ 3.13-era quorum queues did not persist metrics_labels, so there is nothing to recover from disk on restarts.

This approach seems to be the most viable from a few points of view:

1. We are updating the current code only
2. The change is very localized
3. The data structure we want to store into the metrics state is a map with two scalar values (so there's no nesting complexity, for example)

The only alternative is indeed forcing the Ra members to be re-created, which can mean a non-trivial amount of effort for every operator migrating from 3.13 to 4.2 and onwards.

[elo-magnier-7s]

Replied in the other one, I think your objection on the wording is making sense - if that's what you're meaning. And I've proposed a slightly different wording there.

I have some eval (and evil) code that does it in ETS live & rewrites the config file (I've put both the bug report), so it can technically be fixed without doing the whole re-creation of members, but I barely trust it!