title: get the first and last emitted values from an observable
body: <p>Is there a way to get the first and last emitted values from an observable?</p>
  
  <pre><code>const down$ = fromEvent(this.canvas, 'mousedown');
  const up$ = fromEvent(this.canvas, 'mouseup');
  const move$ = fromEvent(this.canvas, 'mousemove');
  const drag$ = move$.pipe(
    skipUntil(down$),
    takeUntil(up$)
  );
  drag$.subscribe((e: MouseEvent) =&gt; {
    console.log(e);
  });
  </code></pre>
  
  <p>Is there a way to get the first and last values from the <pre>drag$</pre> observable?</p>