code: public List<IndexableField> indexableFields(Columns columns) {
          List<IndexableField> fields = new LinkedList<>();
          mappers.values().forEach(mapper -> fields.addAll(mapper.bestEffortIndexableFields(columns)));
          return fields;
      }
docs: Returns the Lucene {@link IndexableField}s resulting from the mapping of the specified {@link Columns}. <p> This
  is done in a best-effort way, so each mapper errors are logged and ignored.
  
  @param columns the {@link Columns} to be added
  @return a list of indexable fields