code: public ServletContextHandlerBuilder createRootServletContextHandler(String subPath, RequestLog requestLogger) {
          if (requestLogger == null) {
              throw new RuntimeException("RequestLogger cannot be null");
          }
          return createRootServletContextHandlerInternal(subPath, requestLogger);
      }
docs: Example requestLogger:
  <pre>
  NCSARequestLog requestLog = new NCSARequestLog("logs/my-app-jetty-web-yyyy_mm_dd.request.log");
  requestLog.setRetainDays(90);
  requestLog.setAppend(true);
  requestLog.setExtended(false);
  requestLog.setLogTimeZone("Europe/Oslo"); // or GMT
  </pre>
  https://wiki.eclipse.org/Jetty/Howto/Configure_Request_Logs