[receiver/k8scluster]: Support list of namespaces to be observed

[bacherfl]

Component(s)

receiver/k8scluster

Is your feature request related to a problem? Please describe.

Currently, the k8scluster receiver config supports setting a single namespace to be observed by the component.
As discussed in #39677 (comment), the flexibility of the receiver could be enhanced to instead take a list of defined namespaces, to observe multiple namespaces in which the collector has read permissions.
This would also better align the config of the k8scluster receiver with other k8s components, such as the k8sevents receiver, the k8sobjects receiver, and the k8sobserver extension, where this is also a list.

Describe the solution you'd like

From a user perspective, the config would change from the following:

k8s_cluster:
   namespace: my-namespace

to something like this (the current namespace config option would need to be deprecated):

k8s_cluster:
   namespaces:
     - my-namespace
     - my-other-namespace

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• receiver/k8scluster: @dmitryax @TylerHelmuth @povilasv @ChrsMark

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[github-actions]

Pinging code owners for receiver/k8scluster: @dmitryax @TylerHelmuth @povilasv @ChrsMark. See Adding Labels via Comments if you do not have permissions to add labels yourself. For example, comment '/label priority:p2 -needs-triaged' to set the priority and remove the needs-triaged label.

[bacherfl]

If this is something we want in this component, I can create a PR for this

[ChrsMark]

I'm in favor of adding support for this. It will help with being in alignment with the other components but also provides flexibility to better target which namespaces to monitor in case that more than one are of interest.

@dmitryax @TylerHelmuth @povilasv please share your perspective on this.

[bacherfl]

I have created a draft PR for how this could be done now - this was a bit more complex than anticipated, as i have outlined in the PR description, but wanted to already post the draft to get feedback, as maybe there might be a simpler approach that i missed