About 4 devs and a year and a lot of changes to how it work to make it run at such a low cost at run time. It is an incredible technical achievement actually and should not just be understood as something easy or a simple switch that Hardware only enables.The ray-traced GI in Metro Exodus is great but it's an achievement that has a lot to thank Nvidia R&D for and the extra iron in your gaming box. A single rendering developer or two probably came up with that GI solution in a reasonable amount of time.
They had a huge gdc presentation on it from the 4 devs that I recommend watching.