Count up from 0 to {count}, showing an item every {delay} milliseconds

DelayCounter.cs

public IObservable<int> ObservableCounter(int count, int delay)
{
    return Observable.Range(0, count).Zip(
            Observable.Interval(TimeSpan.FromMilliseconds(delay)), (item, _) => item);
}

This works as well

return Observable.Interval(TimeSpan.FromMilliseconds(delay))
                             .Select((_, index) => index)
                             .Take(count);

How about...

return Observable.Range(0, count)
                .Delay(TimeSpan.FromSeconds(delay));

Although wasn't in a position to test, I think this will work.

public IObservable<int> ObservableCounter(int count, int delay)
{
    return Observable.For(Observable.Range(0, count), i => Observable.Return(i).Delay(TimeSpan.FromMilliseconds(delay)));
}