fix(server): enforce listen-metrics-urls client TLS info when its scheme is https/unixs

[gyuho]

Otherwise, it will fail with

cannot listen on TLS for 127.0.0.1:8080: KeyFile and CertFile are not presented

We should instead explicitly fail fast, with a clear error message.

Also, adding some documentation how to configure TLS for metrics URLs.

c.f., https://github.com/etcd-io/etcd/issues/8060

[k8s-ci-robot]

Hi @gyuho. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 46.15385% with 7 lines in your changes missing coverage. Please review.

Project coverage is 68.86%. Comparing base (debc8fb) to head (0734121).

:exclamation: Current head 0734121 differs from pull request most recent head 22f20a8

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Additional details and impacted files

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[gyuho]

/retest

[k8s-ci-robot]

@gyuho: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to [this](https://github.com/etcd-io/etcd/pull/18186#issuecomment-2173479410): >/retest Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[ivanvc]

/ok-to-test

[gyuho]

/retest

[serathius]

cc @ahrtr

[serathius]

/retest

[gyuho]

/retest

[gyuho]

/retest