Kubelet cert TTL via GaugeFunc

[sambdavidson]

Rewrote the Kubelet client and serving certificate packages to use a GaugeFunc instead of a traditional Gauge. This allows them both to accurately report the TTL of their certificates at read time rather than arbitrary setting the TTL in unrelated threads.

The metric certificate_manager_server_expiration_seconds has been renamed to certificate_manager_server_ttl_seconds and its behavior has changed from reporting an absolute time, to reporting the seconds remaining e.g. date seconds since epoch like 1257894000 -> remaining seconds like 1234.

The behavior of rest_client_exec_plugin_ttl_seconds remains the same it has just become more accurate when read.

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Building off of new client cert metrics (#84382), new serving cert metrics (#84534), and a GaugeFunc implementation (#83830). We now have accurate insights into Kubetlet certificate rotations and TTL. This provides visibility towards efforts of shortening certificate lifetimes.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

- Renamed Kubelet metric certificate_manager_server_expiration_seconds to certificate_manager_server_ttl_seconds and changed to report the second until expiration at read time rather than absolute time of expiry.
- Improved accuracy of Kubelet metric rest_client_exec_plugin_ttl_seconds.

/assign @awly @logicalhan @liggitt

[logicalhan]

/lgtm

Thanks for iterating over this!

[liggitt]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, sambdavidson

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• pkg/kubelet/certificate/OWNERS [liggitt]
• staging/src/k8s.io/client-go/OWNERS [liggitt]
• staging/src/k8s.io/component-base/metrics/OWNERS [liggitt]

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