Gitlab Metics Prometheus


I have a local Gitlab ce version 11.0.3-ce.0 running on a docker, together with a Prometheus server and a few other things.
I would like to customize the metrics that Gitlab requests from Prometheus. For that, I know that Gitlab Premium has a feature in its GUI that allows metric customization (Custom Metrics backend and UI for creating and editing queries (!3799) · Merge requests · / GitLab · GitLab)
However, as I understand, what the GUI does is change a file named additional_metrics.yml (Files · master · / GitLab FOSS · GitLab)
This means that it should be possible to change this file manually, in the CE version of Gitlab.
However, I am having trouble finding the file’s location in docker.

Anyone knows where it might be or if it exists at all?
Do I have to create it? How does gitlab fetches the default metrics?
Or is it impossible to have custom metrics without Gitlab Premium?

Thank you,
Emanuel Guerra

Problem Solved:

" gitlab_rails[‘env’] = { ‘prometheus_multiproc_dir’ => ‘/dev/shm’ }"
To my .yml file

And Enabled Metric- Prometheus on Gitlab settings