MB-5177: added port 8091 requests rate stat

Change-Id: I2b77f5bb6d80200d3c28d7a5d293b0e303a899da
Reviewed-on: http://review.couchbase.org/30124
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>