@Override
  public void processRequest(RestRequest restRequest, Callback<Void> callback) {
    Exception exception = null;
    frontendMetrics.securityServiceProcessRequestRate.mark();
    long startTimeMs = System.currentTimeMillis();
    if (!isOpen) {
      exception = new RestServiceException("SecurityService is closed", RestServiceErrorCode.ServiceUnavailable);
    } else if (restRequest == null) {
      throw new IllegalArgumentException("RestRequest is null");
    }
    frontendMetrics.securityServiceProcessRequestTimeInMs.update(System.currentTimeMillis() - startTimeMs);
    callback.onCompletion(null, exception);
  }
Default implementation of {@link SecurityService} for Ambry that doesn't do any validations, but just
sets the respective headers on response.
ambry-frontend/src/main/java/com/github/ambry/frontend/AmbrySecurityService.java