Prometheus in agent mode with better integration

[nissessenap]

When https://github.com/XenitAB/terraform-modules/pull/510 get's merged we will have a basic usage of prometheus agent but it's not ideal when it comes to configuration.

There is a issue open at https://github.com/prometheus-operator/prometheus-operator/issues/3989 and when it's get's solved we hopefully will be able to use the agent mode without having to overwrite container args manually.

I open this ticket to be able to track this TODO in the long run.