We will need to monitor CPU and memory usage closely during load testing. Being aware of how traffic affects both will help us in determining how many dynos we need and what size dynos work best for our application. Librato provides a fantastic dashboard that allows us to monitor all of our key performance metrics. We'll see our dyno and Postgres resource usage, as well as router performance metrics. During our load tests, we'll be able to watch how many requests our application is serving and how it affects resource usage on one screen.
Librato works with any Heroku application. It does not require anything to be installed into the application itself. It gathers data by monitoring our Heroku logs and recording and graphing the data.