feat: add additional labels for pod monitor in Cluster

[ma-ts]

Adds the possibility to add additional labels to just the pod monitor. Currently, this is already possible by setting them on the Cluster object, but then a significant amount of resources get labeled with labels that are not relevant there. This is to support multi-tenant Prometheus situations where there might be separate Prometheus scrapers

Fixes: #4435

[github-actions[bot]]

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[rdegez]

Any chance this one get approval ? :-)

We can't use .spec.monitoring.enablePodMonitor: true for this exact same reason (mutiple prometheus instances in the k8s cluster) and have to manage PodMonitor by ourselves because of the inability to specify a custom prometheus label.