release-25.2: spanlatch: add cluster setting for slow latch request threshold

[angeladietz]

Backport 1/1 commits from #158472.

/cc @cockroachdb/release

---

This adds a new cluster setting
kv.concurrency.slow_latch_request_duration which configures the threshold for logging slow latch acquisitions. Previously, logging would occur after 15 seconds, but now the default is reduced to 5s.

Informs #154271
Release note: None
Epic: None
Part of: CRDB-54843

Release justification: low risk obs improvement to help debug slow latch requests

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[blathers-crl]

❌ PR #160035 does not comply with backport policy

Confidence: high
Explanation: The PR introduces a new cluster setting kv.concurrency.slow_latch_request_duration to adjust the threshold for logging slow latch acquisitions, which has been set to a default of 5 seconds. However, a cluster setting by itself, especially when disabled by default (which is not explicitly confirmed in the PR), does not necessarily gate a new feature. Since the cluster setting appears to modify existing behavior rather than introducing a new feature, and the fix does not directly address stability, security, data corruption, performance regression, incorrect results, or debugging production issues, this PR does not meet the critical bug criteria nor does it conform to the feature flag requirement. Additionally, the 'low risk' release justification does not fit any criteria in the policy for an exception. There is no indication this change marks a breaking backward compatibility issue.
Recommendation: Reconsider the backport as it does not meet critical bug fix criteria and is not gated behind a disabled feature flag. Alternatively, provide stronger justification for why this change is critical and should be backported without a feature flag.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is