release-23.1: sqlstats: fix metadata column in system.transaction_activity

[blathers-crl]

Backport 1/1 commits from #104621 on behalf of @xinhaoz.

/cc @cockroachdb/release

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Previously, the metadata column in system.transaction_activity was being incorrectly written as a the aggregated metadata type for statements. This occurs due to using crdb_internal.merge_stats_metadata function when this function only applies to stmt metadata. This bug meant that the UI did not have access to the txn metadata when querying from system.transaction_activity which contains the in-order stmt fingerprints for the txn that is used to build the txn query. Thus the txn query was showing as unavailable for all txns from this table.

As part of this change, the sql activity updater job will get its AOST clause from its newly added sqlstats testing knobs field to make testing simpler.

Fixes: #103618

Release note (bug fix): On the sql stats pages, the txn query should be available so long as the stmt fingerprints associated with the txn also exist in the payload. In other words the majority of txns should have the txn query available when selecting a recent time period.

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Release justification: bug 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 

[j82w]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @xinhaoz)

[j82w]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @xinhaoz)