title: Get number of filtered docs from Lucene search
body: <p>I am doing a Lucene search with a bunch of filters like this:</p>
  
  <pre><code>searcher.search(query, getFilters(), collector);
  </code></pre>
  
  <p>I would like to find out how many documents matched the query but were filtered out. Is that possible already or do I need to subclass some Lucene class and if so, which one and how?</p>