This isn't new, but I hadn't heard about it until recently during a Google Research talk. PACO started as a personal experiment of the creator to track some behaviours of his that were hard to quantify. The open-source project has gone on to become a great little tool to collect personal app usage and behaviour analytics and even to do usability tests with participants over a period of time, e.g. set up an experiment that collects data from the users of your fitness app to see whether they are engaged with your app over a period of a week.
If you're ever going to do that kind of user research, put this app into your tool chest.
The Design of Influence: How to Craft Products that Move People