Alternate way to avaid deprecated observable.source

[angelaki]

Right now I'm using this helper-method to get the HttpRequest behind an Angular HttpClient request:

export function getRequestByObservable<T>(observable: Observable<T>): Observable<HttpRequest<T>> {
    while (observable.source) { observable = observable.source; }
    return <Observable<HttpRequest<T>>>observable.pipe(
        tap(v => { if (!(v instanceof HttpRequest)) { throw new CError('Could not resolve underlying HttpRequest') } })
    );
}

This works, since all HttpRequests rely on an of(req) (https://github.com/angular/angular/blob/c73520bb74fd05250e1653a07f60d3414e0ed90e/packages/common/http/src/client.ts#L664). Since .source is marked deprecated, how to solve this with upcomging RxJs v8?