code: public UUID addNotificationTarget(Function<Notification, Void> target) {
  
      UUID uuid = UUID.randomUUID();
      if(this.notificationTargets.containsKey(uuid)) {
        throw new RuntimeException("Failed to create notification target.");
      }
  
      this.notificationTargets.put(uuid, target);
      return uuid;
  
    }
docs: Add a target for {@link org.apache.gobblin.metrics.notification.Notification}s.
  @param target A {@link com.google.common.base.Function} that will be run every time
  there is a new {@link org.apache.gobblin.metrics.notification.Notification} in this context.
  @return a key for this notification target. Can be used to remove the notification target later.