title: Count number of live concurrent streams
body: <p>I have a stream of streams. I'd like to know to transform this into a live count of many inner streams are currently live, i.e. not completed or errored.</p>
  
  <p>How would I implement CountLiveStreams?</p>
  
  <pre><code>var source = new Subject&lt;IObservable&lt;Unit&gt;&gt;();
  IObservable&lt;int&gt; count = source.CountLiveStreams();
  </code></pre>
  
  <p>Thanks!</p>