Ttdfjt

catalog_controller_product_view event cannot be used in conjunction with FPC cache since Magento will not raise it after the first visit. That's why I am using controller_front_send_response_before observer as it is described in:
https://magento.stackexchange.com/a/126377/31910

My question is, how can I get current product id in this observer?

Istead of the observer you can use the following method to get the detail of current product

use MagentoFrameworkRegistry;

class yourclassname

 /**
 * @var Registry
 */

 protected $_registry;

 public function __construct(Registry $registry)
 
 $this->_registry = $registry;
 

 public yourfunction()
 
 $currentProduct = $this->_registry->registry('current_product');
 //to get id
 $productId=$currentProduct->getId();
 //do rest of your work
 

The main idea is use a request for your purposes. There is a two cases: path without an url rewrite and with url rewrite. Here is a sample code which parse path from request and obtain a product id:

<?php

namespace VendorModuleObserver;

use MagentoFrameworkEventObserver;
use MagentoFrameworkEventObserverInterface;

/**
 * Class ResponseBefore
 *
 */
class ResponseBefore implements ObserverInterface

 /**
 * @var MagentoCatalogUrlRewriteModelResourceModelCategoryProduct
 */
 private $productUrlRewriteResource;

 /**
 * ResponseBefore constructor.
 *
 * @param MagentoCatalogUrlRewriteModelResourceModelCategoryProduct $productUrlRewriteResource
 */
 public function __construct(
 MagentoCatalogUrlRewriteModelResourceModelCategoryProduct $productUrlRewriteResource
 ) 
 $this->productUrlRewriteResource = $productUrlRewriteResource;
 

 /**
 * @param Observer $observer
 * @return void
 */
 public function execute(Observer $observer)
 
 /** @var MagentoFrameworkAppRequestInterface $request */
 $request = $observer->getRequest();

 // Check is we are on the product page

 if ($request->getParam('id')) 
 // Regular request like `catalog/product/view/id/8`
 $id = $request->getParam('id');
 else 
 // In case url rewrite we should search id in the `url_rewrite` table by path and type
 /** @var string $pathInfo */
 $pathInfo = $request->getPathInfo();
 $preparedPathInfo = ltrim(trim($pathInfo), "/");

 $connection = $this->productUrlRewriteResource->getConnection();
 $table = $this->productUrlRewriteResource->getTable('url_rewrite');
 $select = $connection->select();
 $select->from($table, ['entity_id'])
 ->where('entity_type = :entity_type')
 ->where('request_path LIKE :request_path');

 $result = $connection->fetchCol(
 $select,
 ['entity_type' => 'product', 'request_path' => $preparedPathInfo]
 );
 $id = isset($result[0]) ? $result[0] : null;
 

 // Do something here with parsed id

 return;
 

It's not a complete code, but it should help you to write what a you want.