code: @VisibleForTesting
    final synchronized ImmutableSet<String> getAttributeKeys() {
      if (attributes == null) {
        return ImmutableSet.of();
      }
  
      ImmutableSet.Builder<String> builder = ImmutableSet.builder();
      for (Table.Cell<String, String, Object> cell : attributes.cellSet()) {
        builder.add(cell.getRowKey() + ':' + cell.getColumnKey());
      }
      return builder.build();
    }
docs: Returns the attribute keys contained in the attributes map for the file.