flowinfra: clean up flow stats propagation in row-based flows

[yuzefovich]

Previously, we would attach FlowStats (like max memory usage) to the "stream component" stats object. I don't really understand why that was the case, probably it was due to misunderstanding how tracing works (in particular, the TODOs that are now removed mentioned "flow level span", but we don't need to attach the metadata to a particular tracing span).

This simplifies the code a bit but also simplifies the work on adding region information to ComponentID object.

Epic: None

Release note: 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?

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[cockroach-teamcity]

This change is 

[DrewKimball]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @j82w and @rytaft)

[yuzefovich]

TFTR!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[yuzefovich]

blathers backport 23.1

[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 setting reviewers, but backport branch blathers/backport-release-23.1-106546 is ready: POST https://api.github.com/repos/cockroachdb/cockroach/pulls/106597/requested_reviewers: 422 Reviews may only be requested from collaborators. One or more of the teams you specified is not a collaborator of the cockroachdb/cockroach repository. []

Backport to branch 23.1 failed. See errors above.

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