Use Case for Aggregation Metrics Calculated based on Other Metrics

[roytmana]

Is I mentioned in #5608 it would be great to be able to define metrics as an expression against other metrics (ideally but not necessarily including other calculated metrics) on the same bucket level and if possible able to traverse to parent/child buckets. While this calculation could be done outside of ES, sorting of buckets (such as terms) on these calculated values can only be done as part of ES query

Here is one example:

For health care providers I want to find top 10 providers with highest % of overcharge relative to allowed charges (another variation would be % of overcharge relative to average charge) given charge amount and allowed amount fields in my document.

I would like to be able to define aggregation overchargePct as (totalCharges - totalAllowes) * 100 / totalAllowes and do descending sort on this calculated metric

[colings86]

Closing in favour of #8110