Couchbase Server to export Prometheus metrics itself

Currently we have a demand from a number of customers for Couchbase to supply Prometheus metrics, which we plan to meet with the couchbase_exporter and with monitoring in Autonomous Operator 2.0. 

However this is done through changing the existing REST api output into a prometheus format via the exporter. If for example a change is made to the api then a change will have to be made to the exporter. We would like to remove this dependency by having the metrics be reported directly from ns_server.

The current implementation also requires additional implementation and overheads on the Operator e.g. the exporter running as a sidecar running on each kubernetes pod, general development overhead etc