driver: print benchmark labels
driver: print benchmark labels

Change-Id: Ic4ba5d4cfa67a84f0dd479ede1ace1ee965d85c7
Reviewed-on: https://go-review.googlesource.com/39491
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/benchmarks,golang/benchmarks