code: @Override
      public void handleNotificationSent(final ApnsClient apnsClient, final long notificationId) {
          this.notificationStartTimes.put(notificationId, System.nanoTime());
          this.sentNotifications.increment();
      }
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 MicrometerApnsClientMetricsListener} instances, which should always be used for exactly one client
  @param notificationId an opaque, unique identifier for the notification that was sent