internal/godebug: export non-default-behavior counters in runtime/metrics

internal/godebug: export non-default-behavior counters in runtime/metrics

Allow GODEBUG users to report how many times a setting
resulted in non-default behavior.

Record non-default-behaviors for all existing GODEBUGs.

Also rework tests to ensure that runtime is in sync with runtime/metrics.All,
and generate docs mechanically from metrics.All.

For #56986.

Change-Id: Iefa1213e2a5c3f19ea16cd53298c487952ef05a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/453618
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>