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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.