[Cherry Pick #29979] Fix race condition in Dataflow sampler.

[lostluck]

Cherry pick of #29979 to fix #29987 in 2.54.0

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[clmccart]

LGTM

[github-actions]

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[lostluck]

I suspect the failing runs are more to do with the release branch and not having the dataflow images built for the release yet, than this specific change. I'll be merging this in once this last rerun finishes.