The reason it is not in there is because that is not metadata information provided by the file format (unlike PNG, which has this in the acTL chunk). The only way to know how many frames are in a .webp (as far as I know) is to read the entire file and count each of the frames, then multiply by the frame duration. Generally we don't do that, as it is impossible to know upfront how much data you would have to read, which is a performance problem.