Short-circuits updating metadata of a specific type.

The dynamic portion of the hook name, $meta_type, refers to the meta object type
(post, comment, term, user, or any other type with an associated meta table).
Returning a non-null value will effectively short-circuit the function.

Possible hook names include:

• update_post_metadata
• update_comment_metadata
• update_term_metadata
• update_user_metadata

$check = apply_filters( "update_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $prev_value );