Re-enable metadata save when previewing a post

[talldan]

Description
#14877 removed the saving of metaboxes when the preview button is clicked. It did so due to an issue where changes to a published post would not be reflected in the preview when metaboxes are enabled.

This issue tracks reintroducing that functionality in a way where the preview still works.

[paaljoachim]

Is this issue still relevant? I see a PR has been merged.

[jonaskuske]

@paaljoachim

Yep, still very relevant. The only merged PR I see is the one that disabled saving of metadata, so we still need a solution to re-enable it.

[paaljoachim]

Thank you @jonaskuske Jonas!

Dan @talldan is this something you or someone else could take a closer look at?
I'll ping Greg as well, as he might have some ideas.
@gziolo

[gziolo]

I'll ping Greg as well, as he might have some ideas.

Unfortunately, I don't know much about metadata.

[adamsilverstein]

Some related discussion on how to get this working in this and follow up comments: #16006 (comment) I haven't had a chance to work on this yet.