How to create an IObservable that returns the same instance of the observable for each subscription

SingletonObservable.cs

public class SingletonObservable : IObservable<DateTime>, IDisposable
{
  private readonly Subject<DateTime> _subject = new Subject<DateTime>();
  private bool _isRunning = true;

  public SingletonObservable()
  {
    InstanceCount++;

    Task.Run( async () =>
    {
      while ( _isRunning )
      {
        _subject.OnNext( DateTime.Now );
        await Task.Delay( 100 );
      }
    } );
  }

  public void Dispose()
  {
    _isRunning = false;
  }

  public IDisposable Subscribe( IObserver<DateTime> observer )
  {
    return _subject.Subscribe( observer );
  }

  public static int InstanceCount { get; private set; }
}

[Test]
public void HowToReturnSameObservableImplOnSubscribe()
{
  var singletonObservable = new SingletonObservable();

  var subscription1 = singletonObservable.Subscribe( _ => { } );
  Assert.AreEqual( 1, SingletonObservable.InstanceCount );

  var subscription2 = singletonObservable.Subscribe( _ => { } );
  Assert.AreEqual( 1, SingletonObservable.InstanceCount );
}