code: @Override
    public Long llen(final byte[] key) {
      checkIsInMultiOrPipeline();
      client.llen(key);
      return client.getIntegerReply();
    }
docs: Return the length of the list stored at the specified key. If the key does not exist zero is
  returned (the same behaviour as for empty lists). If the value stored at key is not a list an
  error is returned.
  <p>
  Time complexity: O(1)
  @param key
  @return The length of the list.