[sdk-platform-java] Investigate how to support `extraHeaders` for gRPC/ HttpJson

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>` 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.

Read HttpJsonMetadata.java in gax-httpjson and Metadata.java in grpc-java first. Investigate the stated use cases for multiple values per key and how extraHeaders or metadata values should be selected. Done means documenting a clear support approach and its expected behavior.

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