Support Prometheus Metrics in the API Client

Currently the [API client function](https://github.com/hashicorp/consul/blob/087247967c2f00ec23b60692de9626ba7d91b805/api/agent.go#L461-L474) to retrieve metrics uses the standard metrics format.

We would like to be able to use the prometheus format metrics and have them be parsed into some struct. The reasoning is that polling the non-prometheus format metrics disappear and we want to poll the prometheus one so that it doesn't happen.

#### Feature Description

Add a `AgentMetricsPrometheus` function (maybe different) name to the api module.

#### Use Case(s)

We are collecting metrics and don't want them to disappear between metrics aggregation intervals.