This adds a counter for the number of times the application forced a
GC by, e.g., calling runtime.GC(). This is useful for detecting
applications that are overusing/abusing runtime.GC() or
debug.FreeOSMemory().

Fixes #18217.

Change-Id: I990ab7a313c1b3b7a50a3d44535c460d7c54f47d
Reviewed-on: https://go-review.googlesource.com/34067Reviewed-by: Russ Cox <rsc@golang.org>