Expose x-envoy-upstream-service-time as a metric

*Title*: *Expose x-envoy-upstream-service-time as a metric*

*Description*:
> I want to be able to attribute latency to each portion of my network. Whilst I have tracing data which may enable me to do this, it would be much easier if I had access to the data from the X-Envoy-Upstream-Service-Time header available as a metric. I have not been able to determine a way to calculate this with the existing exposed metrics.

[optional *Relevant Links*:]
> https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/router_filter#x-envoy-upstream-service-time