public AutoCheckpointer(ChangeFeedObserver observer)
 {
     this.observer = observer ?? throw new ArgumentNullException(nameof(observer));
 }
 public ObserverExceptionWrappingChangeFeedObserverDecorator(ChangeFeedObserver changeFeedObserver)
 {
     this.changeFeedObserver = changeFeedObserver ?? throw new ArgumentNullException(nameof(changeFeedObserver));
 }