Add support for backendRefs-level upstream metrics in Envoy Proxy

*Description*:
Add support for exposing metrics at the backendRef level in the HTTPRoute configuration. This would be extremely helpful for observing traffic distribution and evaluating the effectiveness of weighted canary deployments.

For Example:

weight canary

```
apiVersion: gateway.networking.k8s.io/v1beta1
kind: HTTPRoute
metadata:
name: canary-route
spec:
parentRefs:
- name: my-gateway
hostnames:
- "example.com"
rules:
- matches:
- path:
type: PathPrefix
value: /
backendRefs:
- name: stable-service
port: 80
weight: 90
- name: canary-service
port: 80
weight: 10

```

Metrics should like

```
envoy_cluster_upstream_rq_total{envoy_cluster_name="httproute/default/canary-route/rule/0/default/stable-service"} 9

envoy_cluster_upstream_rq_total{envoy_cluster_name="httproute/default/canary-route/rule/0/default/canary-service"} 1
```

[optional *Relevant Links*:]

https://github.com/envoyproxy/gateway/issues/4717#issuecomment-2825941211