cmd/bench: compute distribution size

Compute the distribution size as part of cmd/bench.

Fixes golang/go#68706

Change-Id: I73e22c53f6341fcce130a80817265b9e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/758120
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Michael Matloob <matloob@google.com>