title: How to watch all FormControls ValueChanges except specific control?
body: <p>I have a form that do a computation whenever a control input value changes.</p>
  
  <p>Here is my <code>form_group</code> looks like:</p>
  
  <pre><code>form_group = this.fb.group({
      control1: [],
      control2: [],
      control3: [],
      ...
      control10: [],
  });
  </code></pre>
  
  <p>I can detect all controls value changes via <code>form_group.valueChanges</code> observable, and do a computation. However i want to exclude some of the control that doesn't need to do such an action.</p>
  
  <p><strong>But there is anyway how to not detect the changes on a particular control?</strong></p>
  
  <p>It is very burdensome to write a <code>form_group.get('controlN').valueChanges</code> and do a computation there. I have 5 or more controls in my form.</p>