Define what's stable metric in kube-state-metrics repo

[CatherineF-dev]

What would you like to be added: Define what's stable metric and add this definition into documentations.

1. metric name can't be changed
2. existing metric labels can't be changed
3. new fields can be added to existing metrics without having to create a new version

Why is this needed:

Describe the solution you'd like

Additional context

https://github.com/kubernetes/kube-state-metrics/issues/1991#issuecomment-1431628553

[logicalhan]

I actually feel like we shouldn't add labels to stable metrics.

[logicalhan]

/triage accepted /assign

[k8s-triage-robot]

This issue has not been updated in over 1 year, and should be re-triaged.

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/remove-triage accepted

[dashpole]

/triage accepted