This hook allows you to add custom meta elements to filter by custom fields.
An example: To add the elements(options) to meta field(custom field) "test" of filter with ID=1
add_filter('_avalon23_get_meta_options', function($meta_optios, $meta_key, $filter_id){
if ($filter_id == 1 AND $meta_key == 'test'){
$meta_optios = array(
'meta_key' => 'Meta title',
'test1' => 'Test 1',
'test2' => 'Test 2',
'test3' => 'Test 3',
'test4' => 'Test 4',
);
}
return $meta_optios;
},10,3);
