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packages/api-stack/api/api-plugin-audit/src/otel/instruments.md

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+# otel/instruments
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+Owns the OpenTelemetry instrument bundle the audit plugin emits to every request lifecycle. Resolving the instruments lazily on first call lets test harnesses install a custom `MeterProvider` BEFORE the production module loads, so the in-memory exporter captures every observation. Without this lazy resolution, the meter would bind to the no-op default provider at import time and tests would see no metrics.
+
+The four instruments cover the entire surface needed to operate the plugin in production: per-request recorded counter (labelled by outcome and sampled flag), excluded-path counter, sink-failure counter, and a duration histogram.
+
+## Features
+
+- Lazy, cached resolution — first call constructs the meter + four instruments; subsequent calls return the same bundle.
+- `resetAuditPluginInstruments()` test hook drops the cache so a new `MeterProvider` can be picked up between tests.
+- Counter and histogram surfaces follow the OTel semantic-conventions naming style (`audit.<noun>_<unit>` for histograms, `<noun>_total` for counters).
+
+## Usage: Basic
+
+```ts
+import { getAuditPluginInstruments } from "@suaveplan/api-plugin-audit/otel/instruments";
+
+const instruments = getAuditPluginInstruments();
+instruments.requestsRecorded.add(1, { outcome: "success", sampled: "true" });
+instruments.middlewareDuration.record(12.4);
+```
+
+## Usage: Advanced
+
+```ts
+import { metrics } from "@opentelemetry/api";
+import { MeterProvider } from "@opentelemetry/sdk-metrics";
+import {
+    getAuditPluginInstruments,
+    resetAuditPluginInstruments,
+} from "@suaveplan/api-plugin-audit/otel/instruments";
+
+const provider = new MeterProvider();
+metrics.setGlobalMeterProvider(provider);
+resetAuditPluginInstruments();
+
+const instruments = getAuditPluginInstruments();
+instruments.sinkFailures.add(1);
+await provider.forceFlush();
+```
+
+## API Reference
+
+- `interface AuditPluginInstruments { meter, requestsRecorded, requestsExcluded, sinkFailures, middlewareDuration }`.
+- `getAuditPluginInstruments(): AuditPluginInstruments` — cached resolver.
+- `resetAuditPluginInstruments(): void` — drops the cache.
+
+## Implementation Notes
+
+The lazy cache is keyed on a single module-level `let` binding; concurrent first-callers see the same instance because the resolver is synchronous. Histogram unit is explicitly declared `ms` so OTel collectors don't have to infer it from the metric name. The labels on `requestsRecorded` are kept low-cardinality (`outcome ∈ {success, error}` × `sampled ∈ {true, false}`) so the cardinality budget for downstream backends stays bounded.