backupccl: elide expensive ShowCreate call in SHOW BACKUP

[adityamaru]

In #88376 we see the call to ShowCreate taking ~all the time on a cluster with
2.5K empty tables. In all cases except SHOW BACKUP SCHEMAS we do not
need to construct the SQL representation of the table's schema. This
results in a marked improvement in the performance of SHOW BACKUP
as can be seen in #88376 (comment).

Fixes: #88376

Release note (performance improvement): SHOW BACKUP on a backup containing
several table descriptors is now more performant

[cockroach-teamcity]

This change is 

[postamar]

This looks promising.

[adityamaru]

TFTR!

bors r=stevendanna

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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[postamar]

Did we really want to merge all these tracing.ChildSpan calls in this PR? These hardly seem desirable.

[adityamaru]

Did we really want to merge all these tracing.ChildSpan calls in this PR?

@postamar On the DR team we're consciously sprinkling these childspans in every method / sub-operation invoked during a job so that it surfaces more information when grabbing a job trace. In particular, we aggregate the time we spend in each childspan so that when a customer comes to us with a support escalation saying "why is my restore/backup slow?" we have a pretty good first step to narrow down on the cause of the slowdown. As #88376 traces can be quite useful in expediently finding the expensive call. Andrei and obs-inf have done a lot of work to make childspans very cheap to use. Do you have any concerns about this general direction we're headed in that I am missing?

[postamar]

Thanks for this explanation! I was assuming that they were left in by mistake. I'm surprised you can't use reflection to auto-generate the span name, wouldn't that be preferable?