code: func (n *Manager) DroppedAlertmanagers() []*url.URL {
  	n.mtx.RLock()
  	amSets := n.alertmanagers
  	n.mtx.RUnlock()
  
  	var res []*url.URL
  
  	for _, ams := range amSets {
  		ams.mtx.RLock()
  		for _, dam := range ams.droppedAms {
  			res = append(res, dam.url())
  		}
  		ams.mtx.RUnlock()
  	}
  
  	return res
  }
docs: // DroppedAlertmanagers returns a slice of Alertmanager URLs.