Display gateload expvars for total_doc_pulled and total_doc_pushed

Change-Id: I1413447c2e40e30e9966d2a4e1b49a9c7a8aec20
Reviewed-on: http://review.couchbase.org/38172
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Li Yang <lichenyang2@gmail.com>