@Override
  public ReturnCode filterCell(final Cell ignored) throws IOException {
    return ReturnCode.INCLUDE;
  }
A filter that will only return the key component of each KV (the value will be rewritten as
empty).
<p>
This filter can be used to grab all of the keys without having to also grab the values.
hbase-client/src/main/java/org/apache/hadoop/hbase/filter/KeyOnlyFilter.java