Choosing Adoption Metrics

What metrics to use will depend on what you want to achieve. General information on usage of Office 365 can be found in the Usage Analytics pack in PowerBI. Available for download for O365 administrators I think. There is a dashboard in there already pre-populated with main metrics, but with the help of PowerBi you can dive deeper into the details. It will give you information on the  number of users in each tool, how often they use it and  how they use it. 

Additionally we use questionaires in Forms to not only measure numbers but also people's perception on whether their work is becoming simpler and more efficient by using O365 - ultimately that is the goal we want to achieve.