Prometheus in agent mode with better integration

[NissesSenap]

When #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 prometheus-operator/prometheus-operator#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.