In many of my dockerized Gitlab instances, the prometheus/data folder was eating up multiple Gigabytes of hard drive space even though I was not using Prometheus at all.
In order to fix this, I first disabled Prometheus in the docker-compose.yml config using
prometheus_monitoring['enable'] = false
Also see How I reduced gitlab memory consumption in my docker-based setup for a detailed explanantion.
After that, you need to restart gitlab in order for the settings change to take effect.
Now you can just delete the Prometheus data folder. Make a backup of the entire gitlab data folder before this step.
Run this command from within your gitlab data folder:
rm -rf prometheus/data