Just go to the Metro(THQ) facebook page, "Like their Page" and click on the free download button. A screen will show with a "reveal key" to show you your steam product key.

Here is a quick video i found on forums showing you how to do it.
http://www.youtube.com/watch?v=D4rHDJjBMuw&feature=youtu.be

I already tried it and it 100% works so enjoy!

https://www.facebook.com/home.php#!/MetroVideoGame?fref=ts