Add some metrics for CoGBK profiling.

[robertwb]

This could be useful in guiding future optimizations.

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[robertwb]

R: @priyansndesai

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[robertwb]

We're seeing the same structural value error on master: https://github.com/apache/beam/actions/runs/8709671441