Ideally post metrics in the Custom/ space

[serverless-mom]

While it's not the end of the world, it's generally better style for inter-operation with New Relic to start all reported metrics with Custom/ so for example Custom/indices/search/average_time_millis for example.

The drawback here is that making this change breaks any currently set up custom dashboards within New Relic, but if you forsee your number of metrics growing at all, I would recommend making this change.