code: public function execute(Observer $observer)
      {
          if ($this->fastlyConfig->isFastlyEnabled() != true) {
              return;
          }
  
          $event = $observer->getEvent();
          $name = $event->getElementName();
  
          /** @var \Magento\Framework\View\Layout $layout */
          $layout = $event->getLayout();
  
          /** @var AbstractBlock $block */
          $block = $layout->getBlock($name);
  
          if ($block instanceof AbstractBlock) {
              $blockTtl = $block->getTtl();
              if (isset($blockTtl)) {
                  // This page potentially has ESIs so as a first cut let's mark it as such
                  $this->response->setHeader("x-esi", "1");
              }
          }
      }
docs: Set x-esi header if block contains ttl attribute
  
  @param Observer $observer