Improve Envoy Ratelimit filter metrics for shadowing

The metrics in the rate limit service should be sufficient for now. CC @nacx

The envoy metric can be improved by introducing an additional `shadowing` flag in the [ratelimit response](https://www.envoyproxy.io/docs/envoy/latest/api-v3/service/ratelimit/v3/rls.proto#envoy-v3-api-msg-service-ratelimit-v3-ratelimitresponse) from the RL service, and use it to expose the shadowing to the RL filter metrics. This can be tracked in a separate issue and addressed later.

_Originally posted by @zhaohuabing in https://github.com/envoyproxy/gateway/issues/7630#issuecomment-3673094602_