Even though gRPC provides some leeway as "Access to metadata is language dependent", we have yet to see an implementation which doesn't support multiple values per key. Common implementations like grpc-c++, grpc-go, grpc-java provide this and we should do this aswell.

Ref: https://grpc.io/docs/what-is-grpc/core-concepts/#metadata

Ref: https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-metadata.md#constructing-metadata