code: func (c *Client) RegisterUsingToken(req RegisterUsingTokenRequest) (*PackedKeys, error) {
  	if err := req.CheckAndSetDefaults(); err != nil {
  		return nil, trace.Wrap(err)
  	}
  	out, err := c.PostJSON(c.Endpoint("tokens", "register"), req)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	var keys PackedKeys
  	if err := json.Unmarshal(out.Bytes(), &keys); err != nil {
  		return nil, trace.Wrap(err)
  	}
  	return &keys, nil
  }
docs: // RegisterUsingToken calls the auth service API to register a new node using a registration token
  // which was previously issued via GenerateToken.