title: Conditional observable call with promise
body: <p>i'm trying to do some stuff with observable with no skills :) 
  I have something like that (pseudo code):</p>
  
  <pre><code>someobservable$.pipe(
    mergeMap(param1 =&gt; {
      const condition = await promise;
      return condition ? return EMPTY : return of(param1);
    ),
    mergeMap(param1 =&gt; do some stuff)
  );
  </code></pre>
  
  <p>So, what i have to do is if condition in first <code>mergeMap</code> is true, don't go to second <code>mergeMap</code>. This code seems to be working. But how can call and wait for promise in first <code>mergeMap</code>? 
  Or is it wrong approach at all?</p>