code: func GetContentLengthAndFingerprint(
  	ctx context.Context, uri string) (int64, string, error) {
  	resp, err := http.Head(uri)
  	if err != nil {
  		return 0, "", err
  	}
  	if resp.TLS == nil || len(resp.TLS.PeerCertificates) == 0 {
  		return resp.ContentLength, "", nil
  	}
  	fingerprint := &bytes.Buffer{}
  	sum := sha1.Sum(resp.TLS.PeerCertificates[0].Raw)
  	for i, b := range sum {
  		fmt.Fprintf(fingerprint, "%X", b)
  		if i < len(sum)-1 {
  			fmt.Fprint(fingerprint, ":")
  		}
  	}
  	return resp.ContentLength, fingerprint.String(), nil
  }
docs: // GetContentLengthAndFingerprint gets the number of bytes returned
  // by the URI as well as the SHA1 fingerprint of the peer certificate
  // if the URI's scheme is https.