code: @Override
    public Long zcard(final byte[] key) {
      checkIsInMultiOrPipeline();
      client.zcard(key);
      return client.getIntegerReply();
    }
docs: Return the sorted set cardinality (number of elements). If the key does not exist 0 is
  returned, like for empty sorted sets.
  <p>
  Time complexity O(1)
  @param key
  @return the cardinality (number of elements) of the set as an integer.