Monitoring using Prometheus

This is related to monitoring Consul Template. The discussion in #570 has converged to sending health data to Consul (issue has been stale since early 2018). I'm proposing to expose metrics using the Prometheus (soon OpenMetrics format)

Here are my thoughts:
- Works well in Kubernetes settings. Prometheus is most common monitoring solution right now.
- We are using Consul Template as a sidecar. Hence, there is no port conflict (which is one of the concerns by @sethvargo [in #570](https://github.com/hashicorp/consul-template/issues/570#issuecomment-202093286)). The http server could be disabled by default, to prevent breaking changes.
- Consul Template can be used just for Vault, in which case there is no Consul to send data to.
- Other HashiCorp products adapt Prometheus as well (e.g. see https://github.com/hashicorp/vault/pull/5308)