sql/rowexec: subject column backfills to admission control

[erikgrinaker]

kv: add txn helpers that configure admission control

This patch adds the functions DB.TxnWithAdmissionControl() and
kv.NewTxnWithAdmissionControl(), which allow the caller to set the
admission control source and priority. The default variants of these
functions use source OTHER which bypasses admission control.

Release note: None

sql/rowexec: subject column backfills to admission control

This patch subjects column backfills to admission control, using normal
priority.

Release note (bug fix): ALTER TABLE [ADD|DROP] COLUMN are now subject
to admission control, which will prevent these operations from
overloading the storage engine.

[cockroach-teamcity]

This change is 

[erikgrinaker]

I see that backups use ROOT_KV rather than FROM_SQL, not sure which one if appropriate here. We should also consider using a lower priority, but existing callers all seem to use normal priority.

[dt]

We should also consider using a lower priority

#79086

[erikgrinaker]

We should also consider using a lower priority

#79086

Cool, thanks. I want to backport this, and 21.2 only has low/normal/high priorities, so we may want to keep the normal priority just to be conservative. But yeah, we should move all of these bulk jobs over to a lower priority from 22.1 onwards.

[ajwerner]

Reviewed 3 of 3 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @dt and @sumeerbhola)

[sumeerbhola]

Reviewed 1 of 2 files at r2.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @dt)

[rafiss]

more of a general comment for the future: it seems like it would be quite beneficial if we had a way to assert/test that admission control was in effect. (i had the same desire for verifying that the TTL job was using admission control)

[erikgrinaker]

TFTRs!

bors r=ajwerner,sumeerbhola

more of a general comment for the future: it seems like it would be quite beneficial if we had a way to assert/test that admission control was in effect.

Yeah, we need a more comprehensive approach to admission control for internal traffic. I'll write up an issue for this, including testing aspects.

EDIT: #79212.

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 1d4a1a3 to blathers/backport-release-21.2-79115: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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