Prometheus --web.external-url should be configurable

[mmiglier]

Feature Request

Is your feature request related to a problem? Please describe:

Currently I cannot expose Prometheus behind a reverse proxy using /prometheus as prefix.

Describe the feature you'd like:

I would like the default --web.external-url to be overwritten when added to monitoring addiotional_args. Example usage:

  additional_args:
  - --web.external-url=https://example.org/prometheus

Why the featue is needed:

Exposing a service behind a reverse proxy with a different url is a common scenario.

Describe alternatives you've considered:

There is no alternative besides manually updating the run_prometheus.sh after every tiup run.

Teachability, Documentation, Adoption, Migration Strategy: