code: func (s *Server) clustersFromSnapshot(cfgSnap *proxycfg.ConfigSnapshot, token string) ([]proto.Message, error) {
  	if cfgSnap == nil {
  		return nil, errors.New("nil config given")
  	}
  	// Include the "app" cluster for the public listener
  	clusters := make([]proto.Message, len(cfgSnap.Proxy.Upstreams)+1)
  
  	var err error
  	clusters[0], err = s.makeAppCluster(cfgSnap)
  	if err != nil {
  		return nil, err
  	}
  
  	for idx, upstream := range cfgSnap.Proxy.Upstreams {
  		clusters[idx+1], err = s.makeUpstreamCluster(upstream, cfgSnap)
  		if err != nil {
  			return nil, err
  		}
  	}
  
  	return clusters, nil
  }
docs: // clustersFromSnapshot returns the xDS API representation of the "clusters"
  // (upstreams) in the snapshot.