Add replica_role label to metrics for HA setups

[IrvingMg]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds replica_role label to metrics to indicate whether they were emitted by the leader, a follower, or a standalone Kueue instance (default when leader election is disabled).

Which issue(s) this PR fixes:

Closes #6772

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Metrics now include `replica_role` label to identify Kueue instance roles (leader/follower/standalone).```

[k8s-ci-robot]

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[mimowo]

/hold
Let's don't mix the two at the PR level.

Please start with logs only PR. It will be much easier to review.

Once we have the mechanism the follow up for metrics should be easy to digest.

[IrvingMg]

Please start with logs only PR. It will be much easier to review.

I’ve created #8107 for logs only. Once that’s ready, I’ll update the current PR with the metrics.

[IrvingMg]

I’ve created #8107 for logs only. Once that’s ready, I’ll update the current PR with the metrics.

The current PR has been updated and is now ready for review. It includes only the metrics-related changes.

[IrvingMg]

/unhold

/cc @mimowo