View Source Rationale
The design proposed by Telemetry.Metrics might look controversial - unlike most of the libraries available on the BEAM, it doesn't aggregate metrics itself, it merely defines what users should expect when using the reporters.
If Telemetry.Metrics would aggregate metrics, the way those aggregations work would be imposed on the system where the metrics are published to. For example, counters in StatsD are reset on every flush and can be decremented, whereas counters in Prometheus are monotonically increasing. Telemetry.Metrics doesn't focus on those details - instead, it describes what the end user, operator, expects to see when using the metric of particular type. This implies that in most cases aggregated metrics won't be visible inside the BEAM, but in exchange aggregations can be implemented in a way that makes most sense for particular system.
Finally, one could also implement an in-VM "reporter" which would aggregate the metrics and expose them inside the BEAM. When there is a need to swap the reporters, and if both reporters are following the metric types specification, then the end result of aggregation is the same, regardless of the backend system in use.