Update document to reflect Prometheus HTTP Service Discovery

Prometheus HTTP Service Discovery was added to Server in 7.2.1. We need to update the docs to reflect the change. And also update the release notes for 7.2.1.

Notes are here: https://docs.google.com/document/d/1xyTQO24dTFsFytsrSlh5oXMm_Ug0XAn6eMpItBStvSg/edit?usp=sharing