title: Ag-grid: Count the number of rows for each filter choice
body: <p>In my ag-grid I want to display <strong>counts of rows</strong> next to each filter choice in a set filter, and maybe <strong>sort</strong> choices by that count (descending).
  This is what it looks like by default:</p>
  
  <p><a href="https://i.stack.imgur.com/O77z8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/O77z8.png" alt="enter image description here"></a></p>
  
  <p>I want the choices to be displayed as </p>
  
  <ul>
  <li>Select All <strong>(88)</strong></li>
  <li>Katie Taylor <strong>(2)</strong></li>
  <li>Darren Sutherland <strong>(1)</strong></li>
  <li>John Joe Nevin <strong>(1)</strong></li>
  <li>Barack Obama <strong>(0)</strong></li>
  <li>...</li>
  </ul>
  
  <p>What is the most efficient way to get those counts (and maybe sort the choices accordingly) from the row data, taking into account filters already set in the other fields (if any)?</p>