src/Service/ResponseConfigurator.php
protected function setEtag(Response $response)
    {
        if (!$response->getEtag()) {
            $response->setEtag($this->key_builder->getEtag($response));
        }

        return $this;
    }
protected function setEtag(Response $response)
    {
        if (!$response->getEtag()) {
            $response->setEtag($this->key_builder->getEtag($response));
        }

        return $this;
    }
Set ETag.

Need set Last-Modified before ETag

@param Response $response

@return ResponseConfigurator