feat: add total slot ms to RowIterator

[Genesis929]

We need this for BigFrames, see #2234
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Fixes #2234

[chalmerlowe]

@Genesis929

Github indicated that my review was requested, however...

it might just be a bit over eager...

• the PR is still marked as draft
• there is no description outlining why this change is being suggested

As soon as this PR has a description and moves out of Draft status, please let me know and I will gladly take a second look.

So far, everything I see seems good.

[Genesis929]

@Genesis929

Github indicated that my review was requested, however...

it might just be a bit over eager...

• the PR is still marked as draft
• there is no description outlining why this change is being suggested

As soon as this PR has a description and moves out of Draft status, please let me know and I will gladly take a second look.

So far, everything I see seems good.

Thank you very much! Sorry for the trouble, I was going to create a draft, but I accidentally created a pull request, and have to change it back to draft.

[chalmerlowe]

LGTM