allow to specify label sets for which keep_firing_for applies

[calestyo]

Proposal

Hey.

When setting keep_firing_for for some alert rule, it would be nice if one could further select label sets for which it will actually apply.

Like assume one has an alert where for some instances one knows keep_firing_for is needed because they're prone to flapping, but other instances are not, it would be good if one could do something like a instance=~'some pattern' to specify which instances the keep_firing_for should apply to, while leaving all other alone.

Sure one can just make two alerts, one with the keep_firing_for and one without, and then filter the different instances in the alert rule expression. But that makes things quit messy, doesn't it?

It might even be nice if one could specify different time periods, based on different label sets.

Thanks,
Chris.