code: public Filter label(@Nonnull String label) {
      Preconditions.checkNotNull(label);
      return new SimpleFilter(RowFilter.newBuilder().setApplyLabelTransformer(label).build());
    }
docs: Applies the given label to all cells in the output row. This allows the caller to determine
  which results were produced from which part of the filter.
  
  <p>Due to a technical limitation, it is not currently possible to apply multiple labels to a
  cell. As a result, a {@link ChainFilter} may have no more than one sub-filter which contains a
  label. It is okay for an {@link InterleaveFilter} to contain multiple labels, as they will be
  applied to separate copies of the input. This may be relaxed in the future.