cmd/bent: add benchmarks for google protobuf

Change-Id: I9b1dbdf2f224f6a64c274d342c5ee1b5d8f1f34b
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/811200
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>