code: func (e *CombinedEtcdClient) Put(version *EtcdVersion, key, value string) error {
  	if version.Major == 2 {
  		v2client, err := e.clientV2()
  		if err != nil {
  			return err
  		}
  		_, err = v2client.Set(context.Background(), key, value, nil)
  		return err
  	}
  	v3client, err := e.clientV3()
  	if err != nil {
  		return err
  	}
  	defer v3client.Close()
  	_, err = v3client.KV.Put(context.Background(), key, value)
  	return err
  }
docs: // Put write a single key value pair to etcd.