Improve documentation for topology metrics

**Old UI or New UI**
New UI, Consul 1.9.2

**Describe the problem you're having**
I can not get the metrics to show up. I configured a Prometheus backend and added it as a metrics proxy in consul server config. Requests from the UI return with status code 200, but the the UI does not display any values. Prometheus is collecting values for the registered services.

**Describe the solution you'd like**
Please update the documentation for this feature with an example configuration for Prometheus. Obviously there must be some setting I am missing. The example referenced for Kubernetes, does mention some necessary steps, but leaves out the prometheus configuration.