code: @Override
      public void handleNotificationSent(final ApnsClient apnsClient, final long notificationId) {
          this.sentNotifications.mark();
          this.notificationTimerContexts.put(notificationId, this.notificationTimer.time());
      }
docs: Records a successful attempt to send a notification and updates metrics accordingly.
  
  @param apnsClient the client that sent the notification; note that this is ignored by
  {@code DropwizardApnsClientMetricsListener} instances, which should always be used for exactly one client
  @param notificationId an opaque, unique identifier for the notification that was sent