Make `OperationGroup` cache its `...Metadata` properties, as those methods actually *move* the metadata (so a second call to these methods would previously always return empty metadata objects).
Make `OperationGroup` cache its `...Metadata` properties, as those methods actually *move* the metadata (so a second call to these methods would previously always return empty metadata objects).

grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift