Draft: Allow to set arbitrary helm values for prometheus and thanos

Version Control Information

Source branch: feature/helm-values Target branch: feature/nix-config

Description

Part-of: #732

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Merge Prerequisites

• MR title (and description) are descriptive
• Code is readable and syntactically correct
• Code is understandable
• Documentation has been updated, if necessary
• Commit messages look good
• Release note file in RST format added in latest commit

As a developer: please do not tick these boxes yourself. As a reviewer: please get yourself a hot cold beverage.