first commit

docs/localization/i18n/i18n-formatter-relative-time/src/observability/observability.md

@@ -0,0 +1,91 @@
+# RelativeTimeFormatter Observability
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+Per-package OpenTelemetry instruments for `@suaveplan/i18n-formatter-relative-time`. Exposes a histogram (`i18n.format.relative_time.duration_ms`) recording wall-clock duration of every `formatRelativeTime` call (tagged with `locale`, `style`, and the resolved `unit`) and a counter (`i18n.format.cache_hit`) tagged with `formatter: "relative_time"` that increments on every cache reuse.
+
+The two instruments are constructed once via `metrics.getMeter` and are lazily initialised on first record so cold-start latency stays low. The module also retains an in-memory sample ring so spec-conformance tests can assert exact attribute payloads without standing up a full SDK exporter.
+
+## Features
+
+- Lazy instrument construction
+- Stable histogram name `i18n.format.relative_time.duration_ms` with attributes `{ locale, style, unit }`
+- Stable counter name `i18n.format.cache_hit` with attributes `{ formatter: "relative_time" }`
+- In-memory sample ring (`relativeTimeDurationSamples`, `cacheHitSamples`)
+- `resetRelativeTimeFormatMetricsState()` clears both arrays so tests stay isolated
+
+## Usage: Basic Example
+
+```typescript
+import {
+    recordRelativeTimeFormat,
+    recordRelativeTimeCacheHit,
+    getRelativeTimeFormatMetricsState,
+    resetRelativeTimeFormatMetricsState,
+} from "@suaveplan/i18n-formatter-relative-time/observability";
+
+resetRelativeTimeFormatMetricsState();
+recordRelativeTimeFormat("en-GB", "long", "minutes", 1.23);
+recordRelativeTimeCacheHit();
+const state = getRelativeTimeFormatMetricsState();
+state.relativeTimeDurationSamples.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 {
+    formatRelativeTime,
+    clearRelativeTimeFormatterCache,
+} from "@suaveplan/i18n-formatter-relative-time";
+import {
+    getRelativeTimeFormatMetricsState,
+    resetRelativeTimeFormatMetricsState,
+} from "@suaveplan/i18n-formatter-relative-time/observability";
+
+describe("metrics emission", () => {
+    it("emits duration histogram and cache-hit counter", { timeout: 5000 }, () => {
+        clearRelativeTimeFormatterCache();
+        resetRelativeTimeFormatMetricsState();
+        formatRelativeTime(60_000, "en-GB");
+        formatRelativeTime(60_000, "en-GB");
+        const state = getRelativeTimeFormatMetricsState();
+        expect(state.relativeTimeDurationSamples).toHaveLength(2);
+        expect(state.cacheHitSamples).toHaveLength(1);
+    });
+});
+```
+
+## API Reference
+
+### `recordRelativeTimeFormat(locale, style, unit, durationMs): void`
+
+Records one observation on the `i18n.format.relative_time.duration_ms` histogram and pushes a `{ attributes: { locale, style, unit } }` entry into the in-memory sample ring.
+
+### `recordRelativeTimeCacheHit(): void`
+
+Adds 1 to the `i18n.format.cache_hit` counter with `formatter: "relative_time"` and pushes a matching entry into the in-memory sample ring.
+
+### `getRelativeTimeFormatMetricsState(): RelativeTimeFormatMetricsState`
+
+Returns the in-memory sample ring (live reference — do not mutate).
+
+### `resetRelativeTimeFormatMetricsState(): void`
+
+Empties both sample arrays in place.
+
+### `RelativeTimeFormatMetricsState`
+
+`{ relativeTimeDurationSamples: FormatMetricSample[]; cacheHitSamples: FormatMetricSample[] }` where `FormatMetricSample = { attributes: Record<string, string | number> }`.
+
+## Implementation Notes
+
+The instruments are built lazily so consumers who never call `formatRelativeTime` (e.g. tree-shaken builds that import only types) 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 `resetRelativeTimeFormatMetricsState` calls (resetting state does not invalidate the meter).
+
+The module deliberately keeps the in-memory sample ring unbounded. Tests should call `resetRelativeTimeFormatMetricsState()` between cases. Long-running production processes should not retain the ring; the slim-core wave (Wave 6) swaps the in-package observability module for the unified one in `@suaveplan/i18n` core. For now, every leaf package owns its own ring per the design doc's per-package shape.