Ability to run Prometheus with feature-flags

[ArthurSens]

We've been adding a few features that are only enabled under a feature flag. Reasons can be a lack of confidence in reliability, or performance, or that the goal is really to experiment with things.

Currently, Prometheus runs without feature flags enabled in Prombench. It would be useful if feature flags could be passed as arguments to Prombench.

[bwplotka]

+100, I would extend /prombench command with some syntax to reference custom Prometheus pod YAML or even custom specific flags later on