runtime: maintain a direct count of total allocs and frees

runtime: maintain a direct count of total allocs and frees

This will be used by the memory limit computation to determine
overheads.

For #48409.

Change-Id: Iaa4e26e1e6e46f88d10ba8ebb6b001be876dc5cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/394220
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>