runtime/metrics: add the last GC cycle that had the limiter enabled

runtime/metrics: add the last GC cycle that had the limiter enabled

This metric exports the the last GC cycle index that the GC limiter was
enabled. This metric is useful for debugging and identifying the root
cause of OOMs, especially when SetMemoryLimit is in use.

For #48409.

Change-Id: Ic6383b19e88058366a74f6ede1683b8ffb30a69c
Reviewed-on: https://go-review.googlesource.com/c/go/+/403614
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>