Use specific SERIESLY option instead of CBMONITOR

When instantiating a Seriesly instance in MetricHelper, use the
SERIESLY parameter instead of CBMONITOR, so we connect to the correct
host when the two settings are different.

Change-Id: I5ff7c35200f958c8aea2b0a827e32547b3bcd5c9
Reviewed-on: http://review.couchbase.org/40375
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>