code: @Override
    public Long exists(final byte[]... keys) {
      checkIsInMultiOrPipeline();
      client.exists(keys);
      return client.getIntegerReply();
    }
docs: Test if the specified keys exist. The command returns the number of keys exist.
  Time complexity: O(N)
  @param keys
  @return Integer reply, specifically: an integer greater than 0 if one or more keys exist,
  0 if none of the specified keys exist.