Steam users can now use its built-in performance measuring tool to get a wealth of information like CPU/GPU/RAM usage, FPS graphs, and more. It is available for immediate use and can be enabled via the 'In Game' tab. https://www.notebookcheck.net/Steam-adds-new-metrics-to-in-game-performance-overlay.1048418.0.html
It's a good addition but still not usable since there is no CPU temperature.