All Metro apps are launched in a hosting process. Thus, the Soluto software would simply look for that particular process EXE to determine if Metro apps are launched.
However, most OS features do not trigger a new hosting process. For example, connecting to a Wi-Fi network, or opening the charms. (Control Panel would get detected as a Metro app, but how many people open Control Panel every day?)
So while it's not perfect, it is possible to make a first stab at measurement.