code: protected void export(Map<String, String> out, RequestContext ctx, RequestLog log) {
          assert exporter != null;
          exporter.export(out, ctx, log);
      }
docs: Exports the necessary properties to {@link MDC}. By default, this method exports all properties added
  to the export list via {@code add*()} calls and {@code <export />} tags. Override this method to export
  additional properties.