Add a comment count plugin

looks good to me

+++ b/prometheus_exporter.module
@@ -18,3 +18,15 @@ function prometheus_exporter_modules_uninstalled($modules, $is_syncing) {
+
+/**
+ * Implements hook_modules_installed().
+ */
+function prometheus_exporter_modules_installed($modules, $is_syncing) {
+  // @todo replace with $is_syncing once 8.9.x is released.
+  if (!\Drupal::service('config.installer')->isSyncing()) {
+    /** @var \Drupal\prometheus_exporter\MetricsCollectorManager $collector_manager */
+    $collector_manager = \Drupal::service('prometheus_exporter.metrics_collector_manager');
+    $collector_manager->syncPluginConfig();
+  }
+}

This fails with Core 8.8.6 as the second parameter is not defined.

The prototype in core/lib/Drupal/Core/Extension/module.api.php (8.8.6) is not defining the paramater $is_syncing

/**
 * Perform necessary actions after modules are installed.
 *
 * This function differs from hook_install() in that it gives all other modules
 * a chance to perform actions when a module is installed, whereas
 * hook_install() is only called on the module actually being installed. See
 * \Drupal\Core\Extension\ModuleInstaller::install() for a detailed description of
 * the order in which install hooks are invoked.
 *
 * This hook should be implemented in a .module file, not in an .install file.
 *
 * @param $modules
 *   An array of the modules that were installed.
 *
 * @see \Drupal\Core\Extension\ModuleInstaller::install()
 * @see hook_install()
 */
function hook_modules_installed($modules) {

As a result an im port of an existing configuration (and probably a 'drush en prometheus_exporter_MODULE' ) fails with

$ drush cim sync
+------------+------------------------------+-----------+
| Collection | Config                       | Operation |
+------------+------------------------------+-----------+
|            | core.extension               | Update    |
|            | prometheus_exporter.settings | Update    |
|            | smtp.settings                | Update    |
+------------+------------------------------+-----------+

 Import the listed configuration changes? (yes/no) [yes]:
 > 

ArgumentCountError: Too few arguments to function prometheus_exporter_modules_installed(), 1 passed and exactly 2 expected in /srv/httpd/d3ug/htdocs/modules/prometheus_exporter/prometheus_exporter.module on line 25 #0 [internal function]: prometheus_exporter_modules_installed(Array)