[Metrics] Add --show-hidden-metrics-for-version CLI arg

[markmc]

Part of #10582 and discussed in #12745

Add some infrastructure to help us deprecate and remove metrics in a less user-hostile way.

Our deprecation process will now be:

1. Deprecate the metric in 0.N.0 - document the deprecation in
   release notes, user-facing docs, and the help text in /metrics
2. Hide the metric in 0.N+1.0 - users can still re-enable the
   metrics using --show-hidden-metrics-for-version=0.N.0 as an
   escape hatch
3. Remove the metric completely in 0.N+2.0

--show-hidden-metrics takes a version string argument so that users cannot fall into the habit of always enabling all deprecated metrics, which would defeat the purpose.

This approach is copied directly from kubernetes/kubernetes#85270

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