monitoring unit lacks a healthCheck for Prometheus

The monitoring unit HelmRelease does not directly produce the prometheus StatefulSet. It produces the intermediate kind: Prometheus which is the one creating the prometheus StatefulSet.

Same for prometheus alertmanager StatefulSet which is produced by a kind: alertmanager resource.

To have a proper readiness check be done for Prometheus, we could introduce an extra monitoring-ready unit, as a dummy unit adding a healthCheck on these two kind: Prometheus and kind: alertmanager resources.

(with Flux 2.8 -- recently released, see !7033 (closed) -- we should be able to have this be done by Flux HelmRelease controller, without the need for any extra unit)