code: func (c *Client) UpsertNodes(namespace string, servers []services.Server) error {
  	if namespace == "" {
  		return trace.BadParameter("missing node namespace")
  	}
  
  	bytes, err := services.GetServerMarshaler().MarshalServers(servers)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  	args := &upsertNodesReq{
  		Namespace: namespace,
  		Nodes:     bytes,
  	}
  	_, err = c.PutJSON(c.Endpoint("namespaces", namespace, "nodes"), args)
  	return trace.Wrap(err)
  }
docs: // UpsertNodes bulk inserts nodes.