xdsclient/csds: fix leaked metadata (#5339)
xdsclient/csds: fix leaked metadata (#5339)


grpc/grpc-go,dfawley/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,dfawley/grpc-go,menghanl/grpc-go,menghanl/grpc-go,stanley-cheung/grpc-go