feat: Add access mode metric for PersistentVolumes

[viragvoros]

What this PR does / why we need it:
This PR adds a kube_persistentvolume_access_mode metric that exposes the access modes of PersistentVolume objects, in a way that mirrors the existing PVC access mode metric.

At the moment, if we want to filter or aggregate PersistentVolumes by access mode (e.g. ReadWriteOnce, ReadWriteMany, ReadOnlyMany), we have to rely on kube_persistentvolumeclaim_access_mode and join through
PVC metrics. That forces us to scrape the PVC access mode metrics even when we only care about PVs, and increases metric cardinality.

By exposing a dedicated PV access mode metric with a small label set, we can:

• filter and aggregate PVs directly by access mode
• avoid scraping PVC access mode metrics in setups where they are not
  otherwise needed,
  while still keeping metric cardinality low.

Details of the change

• Add kube_persistentvolume_access_mode metric in the PV store:
  • creates one sample for each access mode a PersistentVolume lists
  • access_mode label uses the standard Kubernetes access
    modes (ReadWriteOnce, ReadOnlyMany, ReadWriteMany, etc.)
  • metric follows a similar style as the existing PVC access mode metric
• Update internal/store/persistentvolume_test.go
  • add test coverage for the new PV access mode metric
• Update PV metrics documentation

How does this change affect the cardinality of KSM:
Previously, dashboards and alerts that needed PV access mode information had to join against PVC access mode metrics and therefore scrape kube_persistentvolumeclaim_access_mode, which may have higher cardinality in large clusters.
With kube_persistentvolume_access_mode, users can query PV access modes directly from the PV metrics.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: viragvoros / name: Virag Vörös (d01f7e8)

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[dgrisonnet]

/assign
/triage accepted

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[mrueg]

I wonder if it's worth adding a new metric here or enhancing kube_persistentvolume_volume_mode instead?

[viragvoros]

I considered merging access_mode into the existing kube_persistentvolume_volume_mode metric, but I think keeping them separate is the better choice.
volumeMode and accessMode represent different shapes in the Kubernetes API. A PersistentVolume has exactly one volumeMode, while it may have multiple accessModes. Combining them would require emitting one sample per access mode, which means the volume mode metric would no longer represent a single property of the PV.

There is also a cardinality consideration for metric consumers. If only a combined {volumemode, access_mode} metric existed, users who are interested only in access modes would still have to deal with the extra volume mode label. This increases the number of series they need to aggregate over and makes otherwise simple queries (e.g. counting PVs by access mode) more expensive than using a dedicated access mode metric.

Users who need both dimensions can still correlate them via PV labels.