enhancement(transforms): Add internal metric to record buffer utilization

[bruceg]

Summary

This builds on the work in #24272 and adds support for transform buffer utilization. Since these are input buffers they omit the output label used by the buffers created by the source senders.

Vector configuration

N/A

How did you test this PR?

Ran a quick test dumping internal metrics to stdout and verified that the transform_buffer_* metrics appeared with the appropriate labels.

Change Type

• Bug fix
• New feature
• 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

Notes

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[thomasqueirozb]

Thanks!