This ensures the metrics proxy endpoint is ACL protected behind a
wildcard `service:read` and `node:read` set of rules. For Consul
Enterprise these will need to span all namespaces:
```
service_prefix "" { policy = "read" }
node_prefix "" { policy = "read" }
namespace_prefix "" {
service_prefix "" { policy = "read" }
node_prefix "" { policy = "read" }
}
```
This PR contains just the backend changes. The frontend changes to
actually pass the consul token header to the proxy through the JS plugin
will come in another PR.
