code: protected function setEtag(Response $response)
      {
          if (!$response->getEtag()) {
              $response->setEtag($this->key_builder->getEtag($response));
          }
  
          return $this;
      }
docs: Set ETag.
  
  Need set Last-Modified before ETag
  
  @param Response $response
  
  @return ResponseConfigurator