/**
 * Enables the Custom Fields metabox for reviews
 *
 * To use this snippet, paste it in your active theme's functions.php file.
 *
 * @return void
 */
add_action('admin_init', function () {
    add_post_type_support('provider', 'custom-fields');
});

/**
 * Unprotect Site Reviews' hidden meta keys used for assigned rating counts
 *
 * Use this if you have cloned a page that had assigned reviews, this will
 * allow you to remove the obsolete meta keys with the Custom Fields MetaBox.
 *
 * @param bool $protected
 * @param string $metaKey
 * @return bool
 */
add_filter( 'is_protected_meta', function( $protected, $metaKey ) {
    return !in_array( $metaKey, ['_glsr_average', '_glsr_ranking', '_glsr_reviews'] )
        ? $protected
        : false;
}, 10, 2 );