In a post on the official blog, Metro: Last Light developers 4A Games informs us about the decision to cut out the multiplayer component of the game to focus all resources on single player.
"Throughout the development of Metro: Last light a small, dedicated team had been working on a number of multiplayer prototypes. After E3, we decided to fold this multiplayer team back into the main group and focus 100% of the studio's resources on the single player campaign. As a result, Metro: Last Light will not ship with a multiplayer component."
The developer won't rule out multiplayer as DLC at a later date, but the game won't ship with a multiplayer component.