0302 — pkg/ depends on the OTel metric API only, never the SDK or an exporter
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Context. The metrics integration needs to expose gauges and counters, but importing the OTel SDK or a vendor exporter (Prometheus, Datadog) into the library would add heavy transitive dependencies and pick a vendor on behalf of every consumer.
Decision. The integration accepts a metric.Meter from go.opentelemetry.io/otel/metric — the API package only — defaulting to the global no-op provider, so an unconfigured caller pays zero cost. Each DB-truth number gets an ObservableGauge whose callback re-runs the queue-state query; the in-process AbandonedRoutines numbers use Counter/UpDownCounter. otelexportlab is the one place in the entire repo that imports otel/sdk or a concrete exporter, by design. Precedent: River’s rivercontrib/otelriver — API-only in the core module, vendor integration outside it.
Consequences. Callers connect Prometheus, Datadog, or nothing; pkg/ never knows which. One gotcha the export lab surfaced: ObservableGauges report every collection cycle via their callback, but Counter/UpDownCounter/Histogram emit no data point until something calls .Add()/.Record() — a lab must actually induce each event (here, a hard-timeout abandon) before the instrument appears on the wire. All 13 instruments were confirmed present on a scraped Prometheus body.