You’ve written a great Apps Script library and you want to know how many people are using it, and perhaps even which parts of the library they are using, and how often. Perhaps you have a new version out, and need to know whether anybody is still using the old version. You may also have a set of libraries and you want to know whether users have flipped to the new version. All these things are important to know, but impossible to find out using the Apps script platform as is.
Bruce Mcpherson provides a detailed solution for tracking Google Apps Script library usage using the Properties Service. The source post contains everything you need to integrate this solution into your existing libraries as well as a quick way to chart usage.