release-24.3: security: cache certificate expiration metrics as pointers

[blathers-crl]

Backport 1/1 commits from #142682 on behalf of @angles-n-daemons.

/cc @cockroachdb/release

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security: cache certificate expiration metrics as pointers

Changes in #130110 were added to add labelled ttl metrics to client certificates. It achieved this by changing the system which cached certificate expiries to cache on a composite struct of two metrics, rather than just an expiration metric.

The struct itself housed the metrics as inline values, rather than pointers, so updates were registered in the cached values only, and not the registry in which they were reporting. This means that updates to client certificate expirations would not be reflected by the ttl or expiration metrics.

This ticket modifies those elements so that they are not copied when they are pulled from the cache.

Fixes: #142681
Epic: CRDB-40209

Release note (bug fix): Fixes bug in client certificate expiration metrics.

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Release justification: fixes an a bug for customer certificate authentication

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is