I can't use dpq() with upsert

This is because the Upsert::preExecute() method is protected.

If you use the upsert method and try to see the query with the devel function dpq() you get one error.

    $query = $this->connection->upsert('forcontu_database_counter')
              ->fields([
        'route' => $route,
        'uid' => $uid,
        'user_count' => 1,
        'lastcount' => time(),
      ]);
    $query->key('route');
    $query->execute();
    
    dpq($query);

Error: Call to protected method Drupal\Core\Database\Query\Upsert::preExecute() from context '' in dpq() (line 419 of /Users/adriancid/Sites/drupal82-test/modules/devel/devel.module) #0

But if you set to public the preExecute method you will see:

Error: Call to undefined method Drupal\Core\Database\Driver\mysql\Upsert::arguments() in dpq() (line 424 of /Users/adriancid/Sites/drupal82-test/modules/devel/devel.module) #0