An in-process :telemetry provider: a collector over Telemetry.Metrics definitions, one push feed per metric (spec/08 §11.6, D-140).

start_link(metrics: definitions) attaches one handler for the definitions' events and collects. A definition is read by its fields — name, event_name, measurement, tags, tag_values, keep, unit, description — and its struct's name, so this library takes no dependency on telemetry_metrics. :telemetry itself is optional: without it the module compiles and the provider is never started.

Each metric is a feed (feeds/1, feed/2) whose rows are %{t: DateTime, value: v} plus the metric's tags, and whose schema says so: t the window field, a :time field of DateTimes; value labelled with the definition's description (else its name), in the definition's unit as a UCUM code (Visualize.Unit.from_telemetry/1) with the quantity it implies, each tag a category. A counter's value is the count so far for its tags and a sum's the running total; a last value's, a summary's and a distribution's the measurement.

The collector stamps every row on receipt with its own clock, strictly increasing, and keeps the newest keep: rows of each metric. It honours a window of DateTime bounds and pushes: every every: milliseconds each subscriber gets the rows new since the last flush, its schema sent first at subscription.