title: Create one-time subscription
body: <p>I need to create a subscription to an <code>Observable</code> that is immediately disposed of when it is first called.</p>
  
  <p>Is there something like:</p>
  
  <pre><code>observable.subscribeOnce(func);
  </code></pre>
  
  <p>My use case, I am creating a subscription in an express route handler and the subscription is being called multiple times per request.</p>