cmd/bench: export runstamp configuration key
cmd/bench: export runstamp configuration key

The runstamp is a nanosecond-precision UTC timestamp that indicates when
a benchmark run started. It exists to help prevent comparing results
from different runs, which are generally applicable to compare (machine
conditions, among other things, can change over time).

Change-Id: I5385ba5548033747cbbe9b5f59a5c19b26c3bad3
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/393416
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/benchmarks,golang/benchmarks