title: How to get all attributes(visible on frontend) and their corresponding options of a product collection in Magento?
body: <p>Lets say, these are the attributes color and size.  Color has following options red, green and blue and size has following options small,medium and large.  Now, assume product collection has two products in it and one product is of red color and has size small and other one is of green color and has size medium.  So, I want output like below  </p>
  
  <ul>
  <li><strong>Color</strong>: red,green</li>
  <li><strong>Size</strong>: small,medium</li>
  </ul>
  
  <p>I do not want to fetch attribute list collection first and then iterate and match with it attributes of each and every product in collection because that would be very slow and memory consuming.</p>