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 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