title: rxjs execute tap only at the first time
body: <p>I want to execute tap() only when i get the first emitted value</p>
  
  <p>Something like:</p>
  
  <pre><code>Observable
    .pipe(
       tap(() =&gt; { /* execute only when I get the first emitted value */ })
    )
    .subscribe(() =&gt; {
       // .....
    })
  </code></pre>