fix(metrics): deduct downstream utilization on task transforms

[gwenaskell]

Summary

This PR fixes task transforms utilization reporting by sending a utilization start_wait when the component downstream finishes polling the transform, and a stop_wait when it starts polling again. This ensures that the time elapsed while the transform is idle because the component downstream is busy is reported as idle and not busy, which is already the case for sync/function transforms.

Note: as of now, there is no task transform that spawns a task internally, i.e. they are not run concurrently (https://vector.dev/docs/architecture/concurrency-model/, #11857).
If that becomes a thing in the future, there might be situations where this fix makes the utilization report 0 while the component is doing CPU work, because the component downstream is also busy; but this is already what happens for sync and task transforms.

Vector configuration

How did you test this PR?

Added a unit test that replicates a task transform and measures the utilization in different situations.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Closes: 24641
• Closes: OPA-4673

Notes

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[bruceg]

It took me a couple of reads, because the logic is subtle, but I agree with this approach. A couple small nits below.

[pront]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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[pront]

Hye @gwenaskell, I am going to review this today. Need some time to evaluate future implications of this PR.

[pront]

Code looks good. Left some suggestions for improving the changelog and rustdocs.