code: public function afterGetOutput(\Magento\Framework\View\Layout $subject, $result) // @codingStandardsIgnoreLine - unused parameter
      {
          if ($this->config->getType() == Config::FASTLY) {
              $this->response->setHeader("Fastly-Module-Enabled", "1.2.99", true);
          }
  
          return $result;
      }
docs: Add a debug header to indicate this request has passed through the Fastly Module.
  This is for ease of debugging
  
  @param \Magento\Framework\View\Layout $subject
  @param mixed $result
  @return mixed