feat: Add namespace label to endpoinslice metrics

[mrueg]

What this PR does / why we need it:
endpointslices are namespaced resources and we didn't expose the namespace before.
How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fix #2255
Supersedes #2256

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[CatherineF-dev]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CatherineF-dev, mrueg

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