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0004 — Dual export: Prometheus and OTLP from one snapshot

Status: Accepted

Context

The design spec describes only the Prometheus /metrics path. The exporter-standards family, however, requires every exporter to also push metrics over OTLP, so the same data lands in OpenTelemetry-native backends without a separate scrape. Bolting OTLP on as a second, independently-fetching path would double backend load and risk the two paths disagreeing — a series present on /metrics but absent from the OTLP push, or carrying different label keys. Both paths must therefore derive from the same source of truth.

Decision

Both export paths read the single immutable Snapshot produced by the collection loop (ADR-0001); neither ever touches the backend.

  • Prometheus — an unchecked collector (ADR-0006) whose Collect walks the latest snapshot and emits gauges on each scrape.
  • OTLP — NewOTLPExporter(store, reader) registers one async callback (meter.RegisterCallback) that, on every collection by the SDK reader, reads the latest snapshot and observes the same gauges via Float64ObservableGauge. The reader is injectable: a PeriodicReader wrapping the otlpmetricgrpc exporter in production, a ManualReader in tests.

Per-second values are observable gauges, not counters (ADR-0009): both paths emit identical names, units, and label sets. The label-key consistency invariant (ADR-0006) applies across paths — where the two would otherwise differ, the union label set in canonical order is emitted with empty values for inapplicable keys.

Test invariant: collector tests assert behaviour through both paths — the Prometheus registry Gather and an OTLP ManualReader.Collect — so a regression in either path fails the build.

Consequences

  • One poll per interval per system feeds both export paths; OTLP adds no backend load.
  • The two paths cannot silently diverge: shared snapshot, shared metadata, and a two-path test gate enforce parity.
  • OTLP is push-based and self-paced by the PeriodicReader; Prometheus stays pull-based. Both are at most interval stale, consistent with ADR-0001.
  • Adding a metric means wiring it once into the snapshot projection; both paths pick it up.