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, 1136de4, 4a67b0e)

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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.

[Copilot]

Pull request overview

This PR adds a new PersistentVolume metric (kube_persistentvolume_access_mode) to expose PV access modes directly (mirroring the existing PVC access mode metric), along with test coverage and documentation updates.

Changes:

• Add kube_persistentvolume_access_mode metric generation in the PV store.
• Extend PV store tests to validate emitted access mode samples.
• Document the new PV access mode metric in the storage metrics docs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
internal/store/persistentvolume.go	Introduces a new metric family generator for PV access modes and wires it into the PV metric families list.
internal/store/persistentvolume_test.go	Adds a test case asserting the new metric output for a PV with multiple access modes.
docs/metrics/storage/persistentvolume-metrics.md	Documents the new kube_persistentvolume_access_mode metric in the PV metrics table.

---

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[Copilot]

The docs state that only the four official Kubernetes access modes are emitted, but this implementation emits whatever values appear in p.Spec.AccessModes without filtering. To keep behavior aligned with the docs (and to avoid potentially exposing unexpected/invalid values), restrict emission to the known modes (ReadWriteOnce, ReadOnlyMany, ReadWriteMany, ReadWriteOncePod) or update the docs to match the actual behavior.

Suggested change

	ms = append(ms, &metric.Metric{
	LabelKeys: []string{"access_mode"},
	LabelValues: []string{string(am)},
	Value: 1,
	})
	switch am {
	case v1.ReadWriteOnce, v1.ReadOnlyMany, v1.ReadWriteMany, v1.ReadWriteOncePod:
	ms = append(ms, &metric.Metric{
	LabelKeys: []string{"access_mode"},
	LabelValues: []string{string(am)},
	Value: 1,
	})
	default:
	// Ignore any non-standard or invalid access mode values to align with documented behavior.
	}

[Copilot]

In the metrics table, the access_mode value list is written using raw angle brackets (<...>), which can be interpreted as HTML and render incorrectly. Consider escaping the brackets (</>) to match the formatting used elsewhere in this doc, and ensure the wording matches the actual emission behavior (currently claims only official modes are emitted).

[viragvoros]

Hi @mrueg, thank you for the review. I updated the new metric to match the guidance for newly introduced metrics. Also fixed the docs formatting and removed the sentence that said only the four official access modes are emitted, as the implementation exposes the values in spec.accessModes.