1.16 Release Notes - Add Known CoreDNS metrics regression

[gjtempleton]

As per discussion at office hours.

I've followed the style and syntax as much as possible from upstream k/k given we haven't had known issues blocks since 1.6, open to feedback especially as it doesn't actually apply to all releases in the 1.16 train.

[k8s-ci-robot]

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[johngmyers]

/lgtm
/retest

[johngmyers]

/lgtm cancel
I believe you need to land this into master.

[k8s-ci-robot]

@gjtempleton: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-kops-e2e-kubernetes-aws-1-16	85bf88e	link	/test pull-kops-e2e-kubernetes-aws-1-16

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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[gjtempleton]

Yep, you're right, teaches me for raising these at the end of a long day.