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="<cluster_name>"} in the /stats?format=prometheus endpoint. In the access logs, each log line can be configured to include the response_flags 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.