A series of guides for how to use Prometheus Exporters to obtain data.
Exporter Github: https://github.com/eko/pihole-exporter
This guide isn't going to re-do the steps located on the Github page as they are pretty self explanatory. Basically deploy the docker container version with the correct pi-hole config and then update your prometheus.yml config file for Prometheus with a new scrape job against the pi-hole exporter.
This exporter can also be combined with the InfluxDB script found here.