Description of the Change

This PR changes the query run to detect distinct meta fields, making it so we only get a limited number of fields. It also gets all public fields but only the explicitly allowed private fields.

Closes #3178 and closes #3168

How to test the Change

A site with a big database is needed. Adding the following snippet helps to avoid the transients usage:

add_filter( 'transient_ep_meta_field_keys_post', '__return_false' );
add_filter( 'transient_ep_meta_field_keys', '__return_false' );

Changelog Entry

Changed - Detection of indexable meta fields when visiting the sync and status report pages.

Credits

Props @felipeelia @paoloburzacca

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.