Thanks! The measurement & analytics widgets are not part of CesiumJS. They’re part of the commercial Analytics SDK:
You can use it by getting an on-premise license for Cesium ion engine. Alternatively, if you just need to take measurements of your data, you can upload your data to Cesium ion, and you can use these measuring tools in Cesium Stories. This tutorial explains how to get your data in Stories so you can make distance/area/height measurements:
https://cesium.com/docs/tutorials/stories-introduction/
Let me know if that helps.