Drupal coding standards and practice

I took a minute to make a Merge Request: https://git.drupalcode.org/project/charcount/-/merge_requests/1

You can ignore the patch I made in your issue queue.

From the Drupal API docs:

The class line either gives the default class that provides the service, or if the service uses a factory class, the interface for the service.

For comparison, see this service implementation of the DiffFormatter class in Drupal core:

  /**
   * Creates a DiffFormatter to render diffs in a table.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The config factory.
   */
  public function __construct(ConfigFactoryInterface $config_factory) {
    $config = $config_factory
      ->get('system.diff');
    $this->leading_context_lines = $config
      ->get('context.lines_leading');
    $this->trailing_context_lines = $config
      ->get('context.lines_trailing');
  }

Your $config_factory needs to implement the ConfigFactoryInterface because it is a factory class.