Expose allow-listed Prometheus metrics to end-users

[vlerenc]

End users would benefit from direct access to Prometheus, but we were always reluctant to just open up Prometheus as it also holds "implementation-detail" metrics that end users shouldn't be concerned with (e.g. in relation to internal control plane components that no end user directly interacts with).

According to @wyb1, we have the following options:

1. Expose just allow-listed metrics via proxy: This would have the disadvantages of manually maintaining the allow list and that it wouldn't include Prometheus itself, just the metrics via a proxy.
2. Set up another (end-user) Prometheus scrapping our (main) Prometheus: This would have the disadvantages of manually maintaining the allow list and setting up (TCD) and operating (TCO) another Prometheus instance.
3. Just open up our (main) Prometheus behind a feature gate: This would have the disadvantage, once the feature gate is opened, that all metrics would be exposed.

[wyb1]

Discussion to open up prometheus entirely: gardener/gardener#4800