Custom button to (admin) toolbar

/**
* Implements hook_toolbar().
*/
function MYMODULE_toolbar() {
 $items = [];
 $deploying_domains_count = \Drupal::service('MYMODULE.service')->getDeploingDomains();
 $items['deploying_domains'] = [
   '#type' => 'toolbar_item',
   '#weight' => 999,
   'tab' => [
     '#markup' => "<div class=\"toolbar-icon toolbar-item toolbar-item-custom\">Ready to deploy: $deploying_domains_count</div>",
     '#attributes' => [
       'class' => [
         'toolbar-icon',
       ],
     ],
   ],
   '#cache' => [
     'max-age' => 0,
   ],
 ];
 return $items;
}