helm: add PodMonitor support to ceph-csi-rbd Helm chart

[nepomucen]

Describe what this PR does

This pull request adds support for monitoring the provisioner component of the Ceph-CSI RBD Helm chart via Prometheus by introducing an optional PodMonitor resource.
This enables users to easily integrate metrics collection for the provisioner and its sidecars when Prometheus Operator is installed.

provisioner.podMonitor.enabled=false

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• Reviewed the developer guide on Submitting a Pull
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• Pending release
  notes
  updated with breaking and/or notable changes for the next major release.
• Documentation has been updated, if necessary.
• Unit tests have been added, if necessary.
• Integration tests have been added, if necessary.

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[nepomucen]

@jd would you be so kind and review my PR. This is something we have been patching ever since with strategicMergePatches in our helm release. Would be very nice to have it included in the official helm chart.

[jd]

@jd would you be so kind and review my PR. This is something we have been patching ever since with strategicMergePatches in our helm release. Would be very nice to have it included in the official helm chart.

Not really my thing, sorry.

[nixpanic]

This is quite nice and simple. But the Helm Charts are being deprecated (slowly), and replaced by Ceph-CSI Operator. It probably makes sense to add (optional) support for PodMonitor to Ceph-CSI Operator instead.

Is there a good reason to only add it for provisioners, and not the node-plugins? PodMonitor should be added to other storage types too.