runtime: relate GODEBUG=gctrace output to runtime/metrics

runtime: relate GODEBUG=gctrace output to runtime/metrics

There's more I could list here, but the mapping becomes more complicated
and more fragile. I think this is sufficient to start with.

Fixes #46846.

Change-Id: I6803486a64888b2f38e244b8e2175ad064d648ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/504115
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>