* Revert "Revert "user-agent metadata trait, also: grpc_core::Slice is born (#27770)" (#28108)"
This reverts commit 89d08dad9d.
* will it blend
* will it blend
* will it blend
* lcnagfmt
* sanity
* will it blend
* sleep @ end
* will it blend
* Revert "sleep @ end"
This reverts commit d6bca8ed3d.
* review feedback
* review feedback