release-22.1: kv/concurrency: compute contention event duration from (key,txn) wait start time

[nvanbenschoten]

Backport 1/1 commits from #99166.

/cc @cockroachdb/release

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Fixes #98104.

This commit resolves the bug identified in #98104 where multiple contention events could be emitted with overlapping durations, such that when these durations were added together (e.g. by GetCumulativeContentionTime), their sum was larger than the runtime of the entire query. This was possible because as of 70ef641, we were failing to reset the waiting start time on each new lock holder transaction for the same key.

This commit fixes this by computing the contention event duration using contentionTag.waitStart instead of waitingState.lockWaitStart. It also cleans up some of this code and makes it harder to make such a mistake in the future.

Release note: None

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Release justification: observability fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• 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.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[andreimatei]

Reviewable status: complete! 1 of 0 LGTMs obtained