release-21.2: colfetcher: fix the bytes read statistic collection

[blathers-crl]

Backport 1/1 commits from #75175 on behalf of @yuzefovich.

/cc @cockroachdb/release

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During 21.2 release we adjusted the cFetcher to be Closed eagerly
when it is returning the zero-length batch. This was done in order to
release some references in order for the memory to be GCed sooner;
additionally, the cFetcher started being used for the index join where
the fetcher is restarted from scratch for every batch of spans, so it
seemed reasonable to close it automatically.

However, that eager closure broke "bytes read" statistic collection
since the row.KVFetcher was responsible for providing it, and we were
zeroing it out. This commit fixes this problem by the cFetcher
memorizing the number of bytes it has read in Close. Some care needs
to be taken to not double-count the bytes read in the index join, so
a couple of helper methods have been introduced.

Additionally this commit applies the same eager-close optimization to
the cFetcher when the last batch is returned (which makes it so that
if we've just exhausted all KVs, we close the fetcher - previously, we
would set the zero length on the batch and might never get into
stateFinished).

This commit additionally includes
562d860 which fixes the bytes read
statistic collection for Get requests.

Fixes: #75128.

Release note (bug fix): Previously, CockroachDB could incorrectly report
KV bytes read statistic in EXPLAIN ANALYZE output. The bug is
present only in 21.2.x versions.

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Release justification: low-risk 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 

[yuzefovich]

I also squashed 562d860 into this backport. PTAL.

[rharding6373]

but it looks like the new test is failing CI.

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rytaft and @yuzefovich)

[yuzefovich]

Yep, there were other minor changes on master that this backport needed.