title: rxjs avoid nested subscription with combineLatest
body: <p>I need to fix this nested subscriptions , but i have some difficulties :-(
  could you help me please to find a solution?</p>
  <pre><code>combineLatest([stream1,stream2])
      .pipe(takeUntil(this.destroyed$))
      .subscribe({
        next: ([a, b]) =&gt; {
          this.extService.get(a,b).subscribe();
        },
      });
  </code></pre>