Recording FPS values on games

I'm trying to implement a system where software is collecting FPS information on games. This data would stored and analyzed for statistics. Basically FRAPS without video recording... I don't know where to begin. I tried search for solution to read FPS data on any game but without luck. How can i inject my code to a running process so I know delta time between frames? I guess it has something to do with DirectX and OpenGL specifically but I have no experience in this kind of stuff. How is fraps implemented?