Add response flags to requests metrics

*Title*: Add response flags to request metrics

*Description*:
Request metrics take the form of `envoy_cluster_upstream_rq{envoy_response_code="503",envoy_cluster_name=""}` in the `/stats?format=prometheus` endpoint. In the access logs, each log line [can be configured to include the response_flags](https://www.envoyproxy.io/docs/envoy/latest/configuration/access_log#command-operators) associated with Envoy's response. Adding the `response_flags` that are present in the access log to these metrics would help disambiguate this data and help more quickly determine root causes.