Improve Metrics Definitions

As per DOC-10720, there is an urgent need to improve the quality of the Couchbase Server Metrics Reference.

To make the process of keeping the documentation updated easier, the ask is that each component is able to provide a list of metrics in a consistent format to the docs team, which can simply be imported and styled as necessary with little/no additional effort.

Recognising that each component has their own way of defining their metrics in the source tree, the ask is not to change that, but to be able to produce the list of metrics on demand, ideally as a build artefact that the Docs team can simply extract.

The final format is TBD but the initial suggestion is that each component produces a JSON file with one item per expected metric, where each item contains the metric name/label, the description/help and the expected units/type.