Unreal 2004, quake3, W:ET, etc. Those AAA games already entered the dance and it didn't work out as well as their windows versions. I would not bet a cent on Metro Last Light changing anything to gaming on linux.
It is always "about to change soon" with games on Linux.
Steam for linux is a game-changer though. While previous efforts (there was a surge around 2000 with the likes of Q3, NWN etc) lacked cohesion, there is a convenient banner under which to promote games on linux and real stats on how many people are actually playing games on linux rather than the overhyped estimates of the past.
Linux gaming in the past has too much been about poor-grade clones of popular games. Now via Steam Linux is getting a lot of real games delivered, and in a manner where the publisher/developer can see the true impact of the decisions to port.
So no, Metro: LL might not change anything itself, but it is a sign that other stuff is having a real effect and that there is a pull.
It is always "about to change soon" with games on Linux.