code: public function buildTracker($numItems = Tracker::UNKNOWN, array $extraSubscribers = [])
      {
          $tracker = new Tracker($numItems);
  
          foreach (array_merge($this->defaultSubscribers, $extraSubscribers) as $listener) {
              $tracker->getDispatcher()->addSubscriber($listener);
          }
  
          return $tracker;
      }
docs: Build a new Tracker instance
  
  If $extraSubscribers is not empty, those subscribers will be added
  to the Tracker in addition to the defaults.
  
  @param int                               $numItems          The total number of items (or -1 for unknown)
  @param array|EventSubscriberInterface[]  $extraSubscribers  Optionally specify extra listeners for this Tracker instance
  @return Tracker