Any module can override the default field processor and register its own class for a given field type in hook_field_info_alter() using the tmgmt_field_processor key.
The class must implement \Drupal\tmgmt_content\FieldProcessorInterface.
\Drupal\tmgmt_content\DefaultFieldProcessor
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_field_info_alter(&$info) {
$info['metatag']['tmgmt_field_processor_class'] = '\\Drupal\\Acme\\MetatagFieldProcessor';
}