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+# CurrencyFormatter Observability
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+Per-package OpenTelemetry instruments for `@suaveplan/i18n-formatter-currency`. Exposes a histogram (`i18n.format.currency.duration_ms`) recording wall-clock duration of every `formatCurrency` call (tagged with `locale`, `currency`, and `display`) and a counter (`i18n.format.cache_hit`) tagged with `formatter: "currency"` that increments on every cache reuse.
+
+The currency formatter introduces these instruments as part of the Wave 3 uplift — the monolith currently emits no OTel for currency formatting. The two instruments are constructed once via `metrics.getMeter` and are lazily initialised on first record so cold-start latency stays low.
+
+## Features
+
+- Lazy instrument construction
+- Stable histogram name `i18n.format.currency.duration_ms` with attributes `{ locale, currency, display }`
+- Stable counter name `i18n.format.cache_hit` with attributes `{ formatter: "currency" }`
+- In-memory sample ring (`currencyDurationSamples`, `cacheHitSamples`)
+- `resetCurrencyFormatMetricsState()` clears both arrays so tests stay isolated
+
+## Usage: Basic Example
+
+```typescript
+import {
+    recordCurrencyFormat,
+    recordCurrencyCacheHit,
+    getCurrencyFormatMetricsState,
+    resetCurrencyFormatMetricsState,
+} from "@suaveplan/i18n-formatter-currency/observability";
+
+resetCurrencyFormatMetricsState();
+recordCurrencyFormat("en-US", "USD", "symbol", 0.4);
+recordCurrencyCacheHit();
+const state = getCurrencyFormatMetricsState();
+state.currencyDurationSamples.length; // 1
+state.cacheHitSamples.length; // 1
+```
+
+## Usage: Advanced Example — Asserting OTel emission in a test
+
+```typescript
+import { describe, expect, it } from "@suaveplan/testing/runner";
+import {
+    formatCurrency,
+    clearCurrencyFormatterCache,
+} from "@suaveplan/i18n-formatter-currency";
+import {
+    getCurrencyFormatMetricsState,
+    resetCurrencyFormatMetricsState,
+} from "@suaveplan/i18n-formatter-currency/observability";
+
+describe("metrics emission", () => {
+    it("emits duration histogram and cache-hit counter", { timeout: 5000 }, () => {
+        clearCurrencyFormatterCache();
+        resetCurrencyFormatMetricsState();
+        formatCurrency(42, "USD", "en-US");
+        formatCurrency(42, "USD", "en-US");
+        const state = getCurrencyFormatMetricsState();
+        expect(state.currencyDurationSamples).toHaveLength(2);
+        expect(state.cacheHitSamples).toHaveLength(1);
+    });
+});
+```
+
+## API Reference
+
+### `recordCurrencyFormat(locale, currency, display, durationMs): void`
+
+Records one observation on the `i18n.format.currency.duration_ms` histogram and pushes a `{ attributes: { locale, currency, display } }` entry into the in-memory sample ring.
+
+### `recordCurrencyCacheHit(): void`
+
+Adds 1 to the `i18n.format.cache_hit` counter with `formatter: "currency"` and pushes a matching entry into the in-memory sample ring.
+
+### `getCurrencyFormatMetricsState(): CurrencyFormatMetricsState`
+
+Returns the in-memory sample ring (live reference — do not mutate).
+
+### `resetCurrencyFormatMetricsState(): void`
+
+Empties both sample arrays in place.
+
+### `CurrencyFormatMetricsState`
+
+`{ currencyDurationSamples: FormatMetricSample[]; cacheHitSamples: FormatMetricSample[] }` where `FormatMetricSample = { attributes: Record<string, string | number> }`.
+
+## Implementation Notes
+
+The instruments are built lazily so consumers who never call `formatCurrency` do not pay the OTel meter-lookup cost. The cache check is a strict identity check against the closure-scoped `cached` slot — once built, the instruments are never rebuilt, even across `resetCurrencyFormatMetricsState` calls.
+
+The `currency` attribute is the ISO 4217 code (`"USD"`, `"EUR"`, `"JPY"`); the `display` attribute is the resolved display mode (`"symbol"`, `"code"`, `"name"`, `"narrowSymbol"`). Both are included in the histogram tags so consumers can dashboard the most-formatted currencies and the most-used display modes per locale.