code: public function execute(Observer $observer) // @codingStandardsIgnoreLine - required, but not needed
      {
          if ($this->fastlyConfig->isFastlyEnabled() != true) {
              return;
          }
  
          // If not set, causes issues with embedded ESI
          $this->response->setHeader("x-esi", "1");
      }
docs: Set x-esi header on ESI response request
  If omitted, causes issues with embedded esi tags
  
  @param \Magento\Framework\Event\Observer $observer
  @return void