First, enable access to Prometheus to FAUCET (the Docker instructions in https://github.com/REANNZ/faucet/blob/master/docs/README_install.rst show how to enable access to port 9244).
Then install Prometheus. Under Ubuntu 16.0.4, this is as simple as:
Then configure Prometheus to scrape FAUCET. Under Ubuntu, edit /etc/prometheus/prometheus.yml, and under scrape_configs add a job for FAUCET: