Adds custom attributes to the request object.
\Symfony\Component\HttpKernel\Event\GetResponseEvent $event: The kernel request event.
public function onKernelRequest(GetResponseEvent $event) {
$request = $event
->getRequest();
$request->attributes
->set('custom_attributes', 'custom_value');
$request->attributes
->set('request_attribute', 'request_value');
}