title: How can I measure the duration between 2 actions using redux-observable epic?
body: <p>I'm trying to measure the duration of 2 actions using redux-observable epic. </p>
  
  <p>There is an answer related but it doesn't really help in my situation.
  <a href="https://stackoverflow.com/questions/48183011/in-redux-observable-how-can-i-measure-the-epics-duration-time-when-running-comp">In redux-observable, how can I measure the epics duration time when running complete?</a></p>
  
  <pre><code>testeeEpic = action$ =&gt; action$.ofType('TEST_START', 'TEST_EMD')
    .pipe(
      // How to measure duration between 'TEST_START' and 'TEST_EMD'
    )
  
  /*
  `TEST_START` can be dispatched multiple times before `TEST_EMD`, 
  duration between the lastest `TEST_START` and `TEST_EMD` is needed.
  */
  </code></pre>
  
  <p>It would be grateful if someone can help.</p>