raftstorebench: prevent iter use-after-close

[tbg]

The statsLoop could still be running. In the linked test flake below, we see it call (*pebble).GetMetrics when the engines are already closed.

Rearrange the code so that we wait for statsLoop to terminate properly before exiting the test.

Closes #156138.

Epic: none

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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

[cockroach-teamcity]

This change is 

[arulajmani]

@arulajmani reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @tbg)

[tbg]

The self-hosted runner lost communication with the server. Verify the machine is running and has a healthy network connection. Anything in your workflow that terminates the runner process, starves it for CPU/Memory, or blocks its network access can cause this error.

bors r+

TFTR!

[craig]

Build succeeded:

• linux_amd64_fips_build
• examples_orms
• local_roachtest
• unit_tests
• check_generated_code
• linux_amd64_build
• lint
• acceptance
• docker_image_amd64
• local_roachtest_fips

[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 7880791 to blathers/backport-release-24.3-156288: POST https://api.github.com/repos/tbg/cockroach/merges: 409 Merge conflict []

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

Backport to branch release-24.3 failed. See errors above.

---

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