code: func (s *PresenceService) GetTrustedClusters() ([]services.TrustedCluster, error) {
  	startKey := backend.Key(trustedClustersPrefix)
  	result, err := s.GetRange(context.TODO(), startKey, backend.RangeEnd(startKey), backend.NoLimit)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	out := make([]services.TrustedCluster, len(result.Items))
  	for i, item := range result.Items {
  		tc, err := services.GetTrustedClusterMarshaler().Unmarshal(item.Value,
  			services.WithResourceID(item.ID), services.WithExpires(item.Expires))
  		if err != nil {
  			return nil, trace.Wrap(err)
  		}
  		out[i] = tc
  	}
  
  	sort.Sort(services.SortedTrustedCluster(out))
  	return out, nil
  }
docs: // GetTrustedClusters returns all TrustedClusters in the backend.