[Datatable] When I enable the partial rows, the inspector shows metrics for each bucket

[stratoula]

Kibana version:
7.11 ++

Describe the bug:
I can reproduce this bug on 7.11, so this exists for some versions. The problem is not on the visualization itself but on the inspector.
When I click on the Show partial rows switch and open the inspector, I see extra columns that represent the metrics per bucket.

If I select to export the csv from the inspector, the extra columns are exported which creates some confusion to the users. The inspector should behave as the visualization.

Steps to reproduce:

1. Create a datatable with multiple buckets
2. Enable the partial rows setting
3. Check the inspector

Inspector

This behaves exactly the same with enabling the Show metrics for every bucket/level, so I think the bug must be there.

Expected behavior:
No extra columns should be depicted on the inspector when only the partial rows switch is on.

Any additional context:
Although the inspector csv exporter also exports the additional columns, the visualization exporter works fine.

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[dej611]

This looks very similar to the #121063 bug which I'm fixing on the Lens side.
I suspect the root problem is the same: when passing the datatable over to the download plugin the columns are not filtered based on the visibility.
I can look into this after getting #121802 merged

[stratoula]

Got it. This is not the exact same case. The inspector works fine in all cases except from the one with the partial rows. For example for the Show metrics for each bucket it works fine. If it is off, doesn't show the columns, if it is on it shows them correctly.