xDS metadata keys should be filter instance names, not filter implementation names

In #12274, we changed the keys in `typed_per_filter_config` to match the HTTP filter *instance* name (the [`HttpFilter.name` field](https://github.com/envoyproxy/envoy/blob/7436690884f70b5550b6953988d05818bae3d087/api/envoy/extensions/filters/network/http_connection_manager/v3/http_connection_manager.proto#L1149)) instead of the legacy filter implementation name (e.g., "envoy.filters.http.gcp_authn"). However, the CDS metadata keys are still using the legacy names ([example](https://github.com/envoyproxy/envoy/blob/7436690884f70b5550b6953988d05818bae3d087/source/extensions/filters/http/gcp_authn/gcp_authn_filter.cc#L51)).

I think we should make the same change for CDS metadata keys that we did for `typed_per_filter_config`, so that it becomes possible to specify different metadata for multiple instances of the same filter.