istio: BlackHoleCluster HTTP metrics broken with Telemetry v2

Bug description Since switching to telemetry v2, metrics showing the BlackHoleCluster no longer work (when a service is trying to access a host not defined in its sidecar egress):

bash-5.0$ curl -v http://app.forecourt-service
*   Trying 10.192.42.66:80...
* TCP_NODELAY set
* Connected to app.forecourt-service (10.192.42.66) port 80 (#0)
> GET / HTTP/1.1
> Host: app.forecourt-service
> User-Agent: curl/7.67.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 502 Bad Gateway
< location: http://app.forecourt-service/
< date: Sun, 23 Feb 2020 17:06:29 GMT
< server: envoy
< content-length: 0
<
* Connection #0 to host app.forecourt-service left intact

Metrics:

❯ curl -s http://127.0.0.1:15000/stats/prometheus | grep -i black
envoy_cluster_upstream_cx_total{cluster_name="BlackHoleCluster"} 0
envoy_cluster_upstream_rq_total{cluster_name="BlackHoleCluster"} 0
envoy_cluster_upstream_cx_active{cluster_name="BlackHoleCluster"} 0

Expected behavior The original feature to work as per https://github.com/istio/istio/issues/7669

Steps to reproduce the bug Use telemetry v2, hit a host that is not in your Sidecar egress host

Version (include the output of istioctl version --remote and kubectl version and helm version if you used Helm) 1.4.5

How was Istio installed?

Environment where bug was observed (cloud vendor, OS, etc)