size(key) {
    return new Promise((resolve, reject) => {
      this._client.zcount(key, '-inf', 'inf', (error, size) => {
        if (error) {
          reject(error);
          return;
        }
        resolve(size || 0);
      });
    });
  }
@param {!string} key
@return {!Promise<!number>}
@override