[Smart-Agent] Detectors for Couchbase

[jcabeza]

What is the source and how to collect metrics?
https://github.com/signalfx/signalfx-agent/blob/master/docs/monitors/collectd-couchbase.md

How to monitor?
https://docs.signalfx.com/en/latest/integrations/integrations-reference/integrations.couchbase.html