code: func (k *Key) AsAuthMethod() (ssh.AuthMethod, error) {
  	keys, err := k.AsAgentKeys()
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	signer, err := ssh.NewSignerFromKey(keys[0].PrivateKey)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	if signer, err = ssh.NewCertSigner(keys[0].Certificate, signer); err != nil {
  		return nil, trace.Wrap(err)
  	}
  	return NewAuthMethodForCert(signer), nil
  }
docs: // AsAuthMethod returns an "auth method" interface, a common abstraction
  // used by Golang SSH library. This is how you actually use a Key to feed
  // it into the SSH lib.