code: func (c *Client) Stat(ctx context.Context, key, container string) (size int64, reterr error) {
  	req := newReq(ctx, c.keyURL(container, key))
  	req.Method = "HEAD"
  	c.Auth.SignRequest(req)
  	res, err := c.transport().RoundTrip(req)
  	if err != nil {
  		return 0, err
  	}
  	if res.Body != nil {
  		defer res.Body.Close()
  	}
  	switch res.StatusCode {
  	case http.StatusNotFound:
  		return 0, os.ErrNotExist
  	case http.StatusOK:
  		return strconv.ParseInt(res.Header.Get("Content-Length"), 10, 64)
  	}
  	return 0, fmt.Errorf("azure: Unexpected status code %d statting object %v", res.StatusCode, key)
  }
docs: // Stat Stats a blob in Azure.
  // It returns 0, os.ErrNotExist if not found on Azure, otherwise reterr is real.