After the ValuesSource refactor lands, all aggregation usage will flow through the new registry. This gives us an opportunity to easily count usage of the aggs and expose on the Nodes feature usage API.

The API is helpful for single-purpose endpoints, but endpoints like Search have a huge number of disparate actions which are all lumped together. Searches, aggregations, highlighting, suggestions, scrolling, composite, etc etc all show up under the Search endpoint.

There are many times an administrator would like to see more granular data about what's flowing through their Search endpoint, without enabling slow logs and picking through the firehose. Since queries and aggregations are among the more expensive operations, we should provide more granular feedback on these activities. With a relatively small change to the VS registry we can record how often agg builders are parsed and give an administrator a high-level overview of usage.

It would also make opt-in telemetry simpler and less invasive to collect for XPack features.