new filter to expose memory/cpu/rps to in the response

*Title*: *new filter to expose memory/cpu/rps to in the response*

*Description*:
>Describe the desired behavior, what scenario it enables and how it
would be used.

The ORCA (and client-weighted-round-robin) requires a response to Carry the necessary metrics to help the load balancer to make the decision.

But not all applications have supported that natively. It should be good to support a simple filter to expose some simple metrics the Envoy sidecar could collect to expose it to the gateway.

So, it would be easier for mesh users to give a quick try to the ORCA without much friction.

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.

Start by reading the ORCA and client-weighted-round-robin response requirements and tracing how Envoy filters expose sidecar-collected metrics. Clarify the filter interface and metric definitions before implementation; done means memory, CPU, and RPS are carried in the response so the gateway can use them for load-balancing decisions.

Written by the indexing model from the issue text.