Revert "MB-7483: Remove duplicate stats holding bucket size"

This reverts commit 7e84fc9ce1488722e055a496382af302f655b417.

Change-Id: I889e1bb0411e0c991e061fa3eb7e9300e00e0230
Reviewed-on: http://review.couchbase.org/24669
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>