[kube-monitoring] Show alerts about Prometheus/Thanos issues

[artherd42]

Priority

(Medium) I'm annoyed but I'll live

Is your feature request related to a problem?

Operation teams should "see" when a Prometheus/Thanos is not healthy anymore.

Acceptance Criteria

• The Alert Dashboard shows the alert(s).

[richardtief]

We're going to leverage the blackbox-exporter in the greenhouse namespace to probe ingresses and static targets. The alerts can then be send to Alertmanagers in the respective org namespace.