MB-7354: Provide an aggregated meta data stat

This makes it easier for the ns_server team since they don't
have to do the aggregation themselves.

Change-Id: I5a6f3dfdd63808a88891481d04ad86811be5ad4c
Reviewed-on: http://review.couchbase.org/23618
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>