runtime/metrics: tweak wording of stack and unused memory metrics
This change tweaks and simplifies the descriptions of a couple metrics
to make them easier to parse (for humans).
Change-Id: I852654c7e7042c662ebdfa6334e3baf49ca4b33c
Reviewed-on: https://go-review.googlesource.com/c/go/+/272566
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>