A new metadata type `x-envoy-peer-metadata` is being introduced. We
don't have a better way to do this at the moment compared to just adding
it in `metadata_batch.h`.
The GSM Observability plugin uses this metadata to send topology
information to peers in the form of serialized and base64 encoded
`google::protobuf::Struct`. The individual keys being used inside the
struct are subject to change.
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