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+# Segmenter observability
+
+<!-- begin:auto-docs -->
+_(no submodules)_
+<!-- end:auto-docs -->
+
+## Purpose
+
+The Segmenter formatter exports two OpenTelemetry instruments and a
+small in-memory state ring so spec-conformance tests can assert exact
+attribute payloads without standing up a full SDK exporter. The
+instrument names match the canonical Wave-3 formatter contract —
+every sibling formatter emits the same two metric names so dashboards
+and alerts compose cleanly across the family.
+
+## Features
+
+- Histogram `i18n.format.segmenter.duration_ms` (unit `ms`) — per-segment wall-clock time
+- Counter `i18n.format.cache_hit{formatter="segmenter"}` — increments on cache reuse only
+- In-memory state ring (`getSegmenterMetricsState()`) for spec-conformance
+- `resetSegmenterMetricsState()` test-only helper
+
+## Usage: Basic Example
+
+```ts
+import { metrics } from "@opentelemetry/api";
+import { MeterProvider } from "@opentelemetry/sdk-metrics";
+
+const provider = new MeterProvider({ readers: [/* reader */] });
+metrics.setGlobalMeterProvider(provider);
+
+import { segmentText } from "@suaveplan/i18n-formatter-segmenter";
+segmentText("hello", "en-GB", { granularity: "word" });
+// emits via your reader
+```
+
+## Usage: Advanced Example
+
+```ts
+import { describe, expect, it } from "@suaveplan/testing/runner";
+import {
+    clearSegmenterCache,
+    segmentText,
+} from "@suaveplan/i18n-formatter-segmenter";
+import {
+    getSegmenterMetricsState,
+    resetSegmenterMetricsState,
+} from "@suaveplan/i18n-formatter-segmenter";
+
+it("emits histogram + cache-hit counter", () => {
+    clearSegmenterCache();
+    resetSegmenterMetricsState();
+    segmentText("hello", "en-GB", { granularity: "word" });
+    segmentText("hello", "en-GB", { granularity: "word" });
+    const state = getSegmenterMetricsState();
+    expect(state.durationSamples).toHaveLength(2);
+    expect(state.cacheHitSamples).toHaveLength(1);
+    expect(state.durationSamples[0]?.attributes).toMatchObject({
+        locale: "en-GB",
+        granularity: "word",
+    });
+});
+```
+
+## API Reference
+
+### `recordSegmenterFormat(locale, granularity, durationMs): void`
+
+Records the duration histogram sample. Tags: `locale`, `granularity`.
+Called internally by `segmentText` / `formatSegments`.
+
+### `recordSegmenterCacheHit(): void`
+
+Increments the cache-hit counter with the constant tag
+`{ formatter: "segmenter" }`.
+
+### `getSegmenterMetricsState(): SegmenterMetricsState`
+
+Returns the in-memory ring of `durationSamples` and
+`cacheHitSamples`. Used by spec-conformance tests.
+
+### `resetSegmenterMetricsState(): void`
+
+Clears both ring buffers. Test-only helper.
+
+## Implementation Notes
+
+- The OTel meter is named `suaveplan.i18n.formatter.segmenter`. Every
+  formatter in the family has its own meter so OTel resource
+  attributes can be tied per-package if a consumer chooses.
+- `granularity` defaults to `"grapheme"` in the histogram tag when
+  the caller omits it; `locale` is always required so it's never
+  defaulted.
+- Sample dashboard query (PromQL/Grafana over OTLP):
+  `histogram_quantile(0.99, sum(rate(i18n_format_segmenter_duration_ms_bucket[5m])) by (le, locale, granularity))`
+- The in-memory ring is intentionally unbounded — tests are short
+  and call `resetSegmenterMetricsState()` between cases. A
+  long-running test loop should call reset to avoid memory growth.