rxjs: how can I get informed of an unsubscription of an observable

Question

I have an Angular project where HTTP-searches are started by typing in an input field. This is done like the examples on the web with following code (simplified):

this.form.valueChanges.pipe (
  debounceTime (this.dueTime),
  distinctUntilChanged (compare),
  switchMap (() => async$()
);

function async$ {
    count++;
    return load$ ().pipe (tap ((response: T) => {
        count--;
        ...
      },error => {
        count--;
        ...
      }
    }
}

This works fine, but everytime async$ is started, a counter will be increased and when it is finished, the counter will be decreased again.
So when the subscription is unsubscribed by switchMap, the count is increased but not decreased again, because the success or error method are never called.

So my question is: is there a possibility to get noticed of the cancellation of the subscription so that I can decrease the counter?

It is not an option to put the count logic out of async$ because the function is used elsewhere.