code: func (k *Key) CertUsername() (string, error) {
  	pubKey, _, _, _, err := ssh.ParseAuthorizedKey(k.Cert)
  	if err != nil {
  		return "", trace.Wrap(err)
  	}
  	cert, ok := pubKey.(*ssh.Certificate)
  	if !ok {
  		return "", trace.BadParameter("expected SSH certificate, got public key")
  	}
  	return cert.KeyId, nil
  }
docs: // CertUsername returns the name of the Teleport user encoded in the SSH certificate.