Is your feature request related to a problem? Please describe.
BigFrames (https://cloud.google.com/python/docs/reference/bigframes/latest) uses query_and_wait for query which returns a RowIterator, we need access to slot ms data to properly track it and show it to the user.

Describe the solution you'd like
Add slot ms to RowIterator.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
Additional context
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