runtime/metrics: add /gc/scan/stack:bytes

For #56857

Change-Id: I58187d7c4112b35951014ab14f2969bed7f4c8e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/497319


Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>