@jsm great question! And you’re right, Prometheus.ex lets you define all the custom metrics you want. However, Telemetry tries to achieve another objective.
The goal of Telemetry project is to have a single interface for exposing instrumentation data across the whole ecosystem. Libraries can publish events with this data, and you can subscribe to them and build metrics using these values.
For example, Ecto already uses Telemetry to publish an event on each query. You could subscribe to this event and bump the Prometheus metric without having to instrument every database query yourself. Soon Telemetry events will be also published by Plug and Phoenix.
Today we’ve also released another project, Telemetry.Metrics, which provides a common interface for metric aggregations, like counters, sums, etc. If libraries like Prometheus.ex would integrate with it, setting up metrics which are based on Telemetry events will be even more simple.
Note that neither Telemetry nor Telemetry.Metrics is able to export metrics to external systems. You will always need a library like Prometheus.ex to translate the events to some useful representation 
