Support multiple manually configured Prometheus servers
### Problem to solve
Currently if a user wants to manually deploy Prometheus servers, versus the managed deployments, you can only define a single Prometheus server URL.
### Target audience
- Delaney, Development Team Lead, https://design.gitlab.com/research/personas#persona-delaney
- Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha
- Devon, DevOps Engineer, https://design.gitlab.com/research/personas#persona-devon
- Sidney, Systems Administrator, https://design.gitlab.com/research/personas#persona-sidney
### Further details
<!--- Include use cases, benefits, and/or goals (contributes to our vision?) -->
### Proposal
We should allow a user to manually configure multiple Prometheus servers. To allow this, we will also need to know the environment scope to understand which Prometheus server we should use for a given environment.
To that end, we should:
* Add the option to add additional Prometheus servers, perhaps a model similar to CI Environment Variables (green + sign)
* Add an additional configuration option, the environment scope. This should behave exactly the same as the k8s environment scope.
* If you only have a single Prometheus server, it should have `*` as the environment scope to match all environments. This matches the k8s environment scope functionality.
### What does success look like, and how can we measure that?
Allow end users who need to use manually configured Prometheus servers, to set more than one.
### Links / references
issue