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

[markdroth]

In #12274, we changed the keys in typed_per_filter_config to match the HTTP filter instance name (the HttpFilter.name field) 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).

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.