Model/Layout/LayoutPlugin.php
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;
    }
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;
    }
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