Is there a way to make an in game benchmarking tool like in Ghost Recon Wild Lands?
Ofcorse, i mean all banchmark do is just play graphics and calculate final result based on fps you got 
just make scripted level for benchmark and look on different stats. You can get FPS by dividing 1 to deltatime (1.0/DeltaTime) on tick event (Delta Secounds pin is delta time). Store FPS every or so and at the end of scripted level add it up and divide by numer of results you stored and you will get medium fps result.
As for different stats Blueprint has limited selection of it, you probably will find more of them in C++ aspecially in FPlatform* classes. Generally i recomand to use C++ here so theres less impact of banchmark code on CPU, you can also try blueprint nativization