code: static void addAttachObserver(HTMLElement element, ObserverCallback callback) {
          if (!ready) {
              startObserving();
          }
          attachObservers.add(createObserver(element, callback, ATTACH_UID_KEY));
      }
docs: Check if the observer is already started, if not it will start it, then register and callback for when the
  element is attached to the dom.