title: Subscribe multiple FormControl(s) only once
body: <p>I have filtered out few FormControl(s) from a large FormGroup based on some logic now - I'd like to know how can I merge / combine those selected FormControls and have only one subscribe..</p>
  
  <p>I looked into RxJS docs and marble diagrams but didn't understand it properly. It's very confusing to understand all of these and know which one to use for my case:</p>
  
  <pre><code>- forkJoin
  - combineLatest
  - merge
  - join
  - zip
  </code></pre>
  
  <p>Here's more detailed info about the question:</p>
  
  <p>Let's say 2 FormControls (i.e., controlA, controlB). I want to execute my some <code>handlerFunction</code> whenever any of the FormControl value changes.
  * It needs to execute my handlerFunction once whenever any control valueChanges (In my case any one control will be changed at one time)</p>
  
  <p>Thanks.</p>