code: @Override
    public Long hlen(final byte[] key) {
      checkIsInMultiOrPipeline();
      client.hlen(key);
      return client.getIntegerReply();
    }
docs: Return the number of items in a hash.
  <p>
  <b>Time complexity:</b> O(1)
  @param key
  @return The number of entries (fields) contained in the hash stored at key. If the specified
  key does not exist, 0 is returned assuming an empty hash.