OTLP via the Prometheus bridge over a MetricFamily snapshot

Status

Accepted — implemented in Phase 4. Refines ADR 0002.

Context

The family reference OTLP path (pflex/pstore) collects into a typed snapshot of samples and exposes it through OpenTelemetry observable gauges. idrac_exporter is Prometheus-native: every metric is emitted with prometheus.MustNewConstMetric into a per-target registry. Re-expressing each emitter as an observable gauge would rewrite the entire collector and the on-demand path — contradicting ADR 0002's "reuse the existing collector rather than reimplementing it."

Decision

The optional background loop reuses the existing collectors via a coalescing-safe GatherFamilies(), injects a configurable identity label (default system) and a per-target idrac_up gauge, and publishes an immutable []*dto.MetricFamily snapshot. The OTLP path is go.opentelemetry.io/contrib/bridges/prometheus NewMetricProducer(WithGatherer(store)) feeding a metric.PeriodicReader. No emitter is rewritten. The on-demand /metrics?target= path is unchanged.

Consequences

A dependency on the contrib Prometheus bridge. The snapshot is gathered MetricFamily rather than []Sample; the bridge maps gauges → OTel gauges and counters → monotonic sums (verified). The exporter DOES emit UNTYPED *_info and build_info metrics; labelFamilies converts them to GAUGE (value preserved, labels intact) before they enter the snapshot, so they are exported faithfully via OTLP. Only GAUGE_HISTOGRAM would be dropped by the bridge, which this exporter never emits. The family architecture.md is updated to recognize this "prometheus-native bridge" variant of the OTLP export path.