docs: Update endpointslice-metrics.md

[CatherineF-dev]

What this PR does / why we need it: #2255

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):
Fixes #

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

Resolved by #2266