Create metrics for `service_nodes_total` and `service_nodes_healthy`
Create metrics for `service_nodes_total` and `service_nodes_healthy`

* Register `consul_service_nodes_total` for each service, and
  `consul_service_nodes_healthy` for each service/node combo. Use value of
  `1` for healthy, `0` for unhealthy instead of using labels.
* Add `consul_nodes` as a counter for the number of nodes registered with
  Consul (regardless of services)
* Add `consul_services` as a counter for the number of registered services
* Implement code fixes from PR

clarenceb/consul_exporter,prometheus/consul_exporter