storage: omit MVCCIterator.Stats() call during MVCCScan/MVCCGet

[erikgrinaker]

MVCCScan and MVCCGet attach iterator statistics to an active trace.
However, they would fetch the iterator statistics even if no trace was
active, which has a significant cost. This patch avoids fetching the
stats when there is no active trace.

name                                                                 old time/op    new time/op    delta
MVCCGet_Pebble/batch=true/versions=1/valueSize=8/numRangeKeys=0-24     3.59µs ± 0%    3.34µs ± 0%  -7.02%  (p=0.000 n=10+8)
MVCCGet_Pebble/batch=true/versions=1/valueSize=8/numRangeKeys=1-24     7.43µs ± 1%    7.01µs ± 0%  -5.70%  (p=0.000 n=10+9)
MVCCGet_Pebble/batch=true/versions=10/valueSize=8/numRangeKeys=0-24    4.62µs ± 1%    4.36µs ± 1%  -5.53%  (p=0.000 n=10+10)
MVCCGet_Pebble/batch=true/versions=10/valueSize=8/numRangeKeys=1-24    10.4µs ± 1%    10.0µs ± 1%  -3.79%  (p=0.000 n=10+10)

Resolves #86083.
Touches #64503.

Release note: None

[cockroach-teamcity]

This change is 

[nicktrav]

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

[erikgrinaker]

TFTR!

bors r=nicktrav

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential 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.

---

error creating merge commit from c739468 to blathers/backport-release-21.2-86084: 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.

---

error creating merge commit from c739468 to blathers/backport-release-22.1-86084: 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 22.1.x failed. See errors above.

---

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is otan.}