SimpleAsyncSubject<T> invokes _observers.ToArray() on every event

[alexanderfedin]

The code looks like the following:
public ValueTask OnCompletedAsync()
{
IAsyncObserver[] observers;

        lock (_gate)
        {
            ...
            observers = _observers.ToArray();
        }

        return OnCompletedAsyncCore(observers);
    }

It would be much less memory pressure (in terms of garbage collection) to use ImmutableList to store the subscribers
SimpleAsyncSubject.zip
.