title: Angular 4+ ngOnDestroy() in service - destroy observable
body: <p>In an angular application we have <code>ngOnDestroy()</code> lifecycle hook for a component / directive and we use this hook to unsubscribe the observables.</p>
  
  <p>I want to clear / destory observable that are created in an <code>@injectable()</code> service.
  I saw some posts saying that <code>ngOnDestroy()</code> can be used in a service as well.</p>
  
  <p>But, is it a good practice and only way to do so and When will it get called ?
  someone please clarify.</p>