code: func (c *Client) HTTPVersion(ctx context.Context) (string, error) {
  	req := c.newRequest(ctx, "HEAD", c.discoRoot(), nil)
  	res, err := c.doReqGated(req)
  	if err != nil {
  		return "", err
  	}
  	return res.Proto, err
  }
docs: // HTTPVersion reports the HTTP version in use, such as "HTTP/1.1" or "HTTP/2.0".