code: func (m *Mercury) UpdateNode(ctx context.Context, node *types.Node) error {
  	key := fmt.Sprintf(nodeInfoKey, node.Podname, node.Name)
  	bytes, err := json.Marshal(node)
  	if err != nil {
  		return err
  	}
  
  	value := string(bytes)
  	log.Debugf("[UpdateNode] pod %s node %s cpu slots %v mem %v", node.Podname, node.Name, node.CPU, node.MemCap)
  	_, err = m.Put(ctx, key, value)
  	return err
  }
docs: // UpdateNode update a node, save it to etcd
  // storage path in etcd is `/pod/nodes/:podname/:nodename`