Create k8s-metrics-reviewer.md for SIG-Instrumentation

[CatherineF-dev]

Add SIG-Instrumentation Metrics Review guidelines for reviewing Kubernetes metrics changes, including stability framework and naming conventions.

Which issue(s) this PR fixes:

Fixes #9036

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: CatherineF-dev
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• contributors/devel/sig-instrumentation/OWNERS

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[k8s-ci-robot]

@CatherineF-dev: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-community-verify	ebc12b7	link	true	/test pull-community-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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