mixin: Introduce flexible multi-cluster/namespace mode for alerts and dashboards

[kakkoyun]

This would be useful for users who don't use Kubernetes. In the current state of mixin we assume, there's a namespace label for all metrics. This is a Kubernetes term, it'd be better to be environment agnostic.

cc @metalmatze @brancz

[metalmatze]

Basically we should setup Thanos with systemd once and check that the mixin works as expected.
We could do something similar to the multi cluster mode that we have with the kubernetes-mixin and enable kuberentes in a config.

[brancz]

Aside from removing kubernetes labels .. is there really anything else we can do other than allowing the user to specify what label(s) make two clusters distinct?

[kakkoyun]

We'll definitely let them specify the labels and maybe we can provide a way to introduce different hierarchies that would both work for alerts and dashboards (e.g region, cluster, namespace etc). Basically, let them specify a layer. From that, we can inject a variable, a dropdown to dashboards and inject selectors/aggregators to alerts.

[brancz]

Makes sense 👍

[stale]

Hello 👋 Looks like there was no activity on this issue for the last two months.
Do you mind updating us on the status? Is this still reproducible or needed? If yes, just comment on this PR or push a commit. Thanks! 🤗
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[kakkoyun]

Done.