Add Request Response Size to Cluster Grafana dashboard

[luvk1412]

Enhance grafana dashboard to add Request response sizes Panel (suggested here). Request response sizes histogram metrics can be enabled by setting enabledRequestResponseSizeMetric to true in ProxyMetric once #4314 is merged.

Changes needs to be made in charts/gateway-addons-helm/dashboards/envoy-clusters.json

I have one concern about adding this: first time users will see this panel empty as this stat will be disabled by default. If this behaviour is fine, we can add.

[zirain]

if should be collapsed by default, and a warning message to tell user how to enable it.