public static IObservable <T> AsyncFinally <T>(this IObservable <T> source, Func <IObservable <object> > action) => source.AsyncFinally(async() => await action().ToTask());
public static IObservable <T> AsyncFinally <T>(this IObservable <T> source, Func <IObservable <Unit> > action) => source.AsyncFinally(() => action().ToTask());