release-24.1: ui: fix replication lag metric for multinode clusters and cutover

[blathers-crl]

Backport 1/1 commits from #123510 on behalf of @kev-cao.

/cc @cockroachdb/release

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Replication lag metric would report absurdly high lag for multinode clusters as it would take the average of the reported timestamps, and as some nodes may report 0, this would cause extremely low replicated times. To resolve this, the metric should pick the maximum time reported by all of the nodes. Additionally, on cutover or job fail/cancellation, stop reporting replicated time to avoid falsely reporting high replication lag.

Informs #120652

Release note (ui change): fix replication lag metric reporting for multinode clusters and cutover

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Release justification: current replication lag metric reports incorrectly for multinode clusters

[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.
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• Backports should not add new functionality (except as defined
  here).
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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).
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  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 

[dhartunian]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @kev-cao, @msbutler, and @xinhaoz)