subset lb: create metadata match criteria based on the metadata of route/vhhost

[wbpcode]

Title: subset lb: create metadata match criteria based on the metadata of route/vhhost

The specific metadata_match and dynamic metadata will be used to create metadata match criteria which will be used by the subset lb. But the metadata configured in the route/vhhost is not used for now.

Maybe we can try to merge the route/vhhost metadata that under the envoy.lb namespace?