Add a relabeled alerts metric

[sradco]

Add a metric alerts_effective_active_at_timestamp_seconds, that reports the activate time of the alerts after relabeling, for both user and platform alerts.

This enhancements is related to #1822

[sradco]

/assign @simonpasquier

[openshift-ci]

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[openshift-ci]

@sradco: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/markdownlint	45b8ceb	link	true	/test markdownlint

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[openshift-bot]

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/lifecycle stale

[openshift-bot]

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