private void start() {
      RestRequestMetrics requestMetrics =
          metrics.getStatsReportMetricsGroup.getRestRequestMetrics(restRequest.isSslUsed(), false);
      restRequest.getMetricsTracker().injectMetrics(requestMetrics);
      // Start the callback chain by performing request security processing.
      securityService.processRequest(restRequest, securityProcessRequestCallback());
    }
Start the chain by calling {@link SecurityService#processRequest}.
ambry-frontend/src/main/java/com/github/ambry/frontend/GetStatsReportHandler.java