Support namespace selector for filtering

[cskinfill]

What would you like to be added:
Support using namespace selector for filtering which resources to acquire metrics for.

Why is this needed:
This would seem to fit more naturally into the kubernetes model of using selectors to select resources. Also, this makes it easier to select on namespaces that have a label, rather then trying to manage a list of namespace names.

Describe the solution you'd like
--namespace-selector workload=testme would select all namespaces with the label workload equal to the value testm.
Additional context

[nerzhul]

Hello,
I was about to ask the same thing. Where i work we are moving kube-state-metrics from a global instance to a per client instance, and i'd like to use a namespace selector to discard migrated clients from the global instance (like we can do with prometheus operator).
Is it possible ?

[dashpole]

/assign @dgrisonnet
/triage accepted

[dgrisonnet]

What you are proposing would be to add label selector rather than a namespace selector. We already have a namespace selector with the --namespaces option: https://github.com/kubernetes/kube-state-metrics/blob/main/pkg/options/options.go#L145

What you could do today to have a similar result would be to do:

kubectl get namespaces -l workload=testme

and append the list of namespaces to the --namespaces option of kube-state-metrics.

[AntoineNGUYEN31]

@dgrisonnet your solution works, but cannot be scaled especiallyon on a managed k8s cluster. Admin will be solliciated on very newly created important namespace, and it implies redeployement of the stack.

[Serializator]

Just want to mention this here. This idea could help with #2400 as well. In a limited privilege environment this could help with only listing resources on namespaces the service account is privy too through the use of labels and selector at the namespace level.