Internally, use `std::vector` instead of `ChunkedVector` to hold extra metadatum.
I'm not totally convinced this is the right move, so it's going to be a try it and monitor for a month or so thing... I might roll back if performance is actually affected (but I think we'll see some wins and losses and overall about a wash).
Closes#36118
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36118 from ctiller:YUPYUPYUP 68e0acd0a2
PiperOrigin-RevId: 620902195