Stats: Expose downstream_rq_xx counter with specific response codes

Currently, for the downstream response count, we have downstream_rq_1xx, downstream_rq_2xx metrics which are aggregated by the response codes, and thereby there's only the response code class label exposed. It would be good to have these metrics by specific response code along with aggregating them into a response code class.

There is something similar already exposed in case of [upstream](https://www.envoyproxy.io/docs/envoy/latest/configuration/upstream/cluster_manager/cluster_stats#dynamic-http-statistics) metrics, would be useful to have the same for downstream ones as well.