In Terms Indexablequery_db, for term count, use wp_count_terms() instead of WP_Term_Query

[rebeccahum]

Description of the Change

We're seeing on sites with a very large amount of terms, terms indexing isn't starting because of a query that is being killed:

SELECT t.*, tt.* FROM wp_terms AS t INNER JOIN wp_term_taxonomy AS tt ON t.term_id = tt.term_id WHERE t.term_id IN (.....,......,......,......,......,......,......,......,......,......,......,......,......,......,......,......,......,......,......)

This is because in query_db() for the terms Indexable, we are calling a new instance of WP_Term_Query, which ultimately calls _prime_term_caches and triggers an expensive, unnecessary query: https://github.com/WordPress/WordPress/blob/e99a11600602fbb5282b59227004d61c543d812d/wp-includes/taxonomy.php#L4071

It isn't required for the ultimate term count in $total_objects, so we should be good to replace it with a simple wp_count_terms() call (which doesn't ultimately call _prime_term_caches() due to an early return).

Aside: I've also set hierarchical to false because I'm seeing that it performs a no limit query which, on a site with a lot of terms, is being killed: https://github.com/WordPress/WordPress/blob/e99a11600602fbb5282b59227004d61c543d812d/wp-includes/class-wp-term-query.php#L635-L644. I don't think it's the hierarchical structure is necessary for indexing (correct me if I'm wrong).

How to test the Change

1. Attempt to index a terms indexable and validate that the terms count is the same in wp vip-search index --indexables=term

Changelog Entry

Changed - counts are now calculated with wp_count_terms() in Indexable Terms query_db

Credits

Props @rebeccahum

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.