title: Get expiration time of Rails cached item
body: <p>Somewhere in my app I use</p>
  
  <pre><code>Rails.cache.write 'some_key', 'some_value', expires_in: 1.week
  </code></pre>
  
  <p>In another part of my app I want to figure out how much time it is left for that cache item.</p>
  
  <p>How do I do that? </p>