@Override
  public List<String> getGroupAuths(String[] groups, boolean systemCall) throws IOException {
    assert (labelsRegion != null || systemCall);
    List<String> auths = new ArrayList<>();
    if (groups != null && groups.length > 0) {
      for (String group : groups) {
        Get get = new Get(Bytes.toBytes(AuthUtil.toGroupEntry(group)));
        getAuths(get, auths);
      }
    }
    return auths;
  }
This is a VisibilityLabelService where labels in Mutation's visibility expression will be
persisted as Strings itself rather than ordinals in 'labels' table. Also there is no need to add
labels to the system, prior to using them in Mutations/Authorizations.
hbase-server/src/test/java/org/apache/hadoop/hbase/security/visibility/ExpAsStringVisibilityLabelServiceImpl.java