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, reporter_options — 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 labeled 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.

value says how it was measured (D-160): its metric_kind is :counter for a counter or a sum and :gauge for every other kind. The definition's reporter_options[:visualize] adds what only the application knows of it — exactly polarity, bounds, target and metric_kind, the last overriding the inference:

summary("http.request.duration",
  unit: {:native, :millisecond},
  reporter_options: [visualize: [polarity: :lower_is_better, bounds: [0, nil], target: 250]]
)

The schema checks them, and any other key is refused: an invalid fact stops start_link/1 with {:telemetry_facts, metric, errors}, the metric named as feeds/1 keys it.

The collector stamps every row on receipt with its own clock, strictly increasing, and keeps the newest keep: rows of each metric. It honors 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.