code: func (m *Manager) SavePublicKey(username string, key PublicKey) error {
  	if err := m.RemovePublicKey(username, key.Label); err != nil {
  		return err
  	}
  	return m.getUserCollection().Update(
  		bson.M{"username": username},
  		bson.M{"$push": bson.M{"publickeys": key}})
  }
docs: // SavePublicKey save or update public key along with its label