For HttpJson: https://github.com/googleapis/sdk-platform-java/blob/main/gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/HttpJsonMetadata.java#L43
For gRPC: https://github.com/grpc/grpc-java/blob/master/api/src/main/java/io/grpc/Metadata.java

Metadata is implemented with as a Map<String, List<String>> as a key can have multiple values.

We want to investigate how properly support or expand support for extraHeaders / Metadata values. Look into the use cases for having multiple values in each key and how to pick which value to use.