document LSM based aggregations in APM Server

Document LSM based metrics aggregation in apm-server, once switched to https://github.com/elastic/apm-aggregation

This includes two parts of docs
- more detailed dev docs, living in this repo, explaining how the LSM based aggregator is used in apm-server; based on
- customer facing public docs: update [data-model-metrics](https://www.elastic.co/guide/en/apm/guide/current/data-model-metrics.html#_calculated_metrics) where necessary after switching to LSM based aggregation.