[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.

首先阅读 gax-httpjson 中的 HttpJsonMetadata.java 和 grpc-java 中的 Metadata.java。调查每个键对应多个值的已述用例，以及应如何选择 extraHeaders 或 metadata 值。完成的标准是记录清晰的支持方案及其预期行为。

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