Implement multi-cluster alerts

[jkroepke]

What this PR does / why we need it: Add support for multi cluster prometheus alerts. If the label does not exists on metrics, the behavior is not changed.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[k8s-ci-robot]

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[mrueg]

Thanks!
/approve

Others might want to take a look as well.

[jkroepke]

@mrueg who are the other people?

[CatherineF-dev]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CatherineF-dev, jkroepke, mrueg

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