# Date observability

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## Purpose

The Date 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 (number, relative-time, list, display-name,
currency, collator, segmenter) emits the same two metric names so
dashboards and alerts compose cleanly across the family.

## Features

- Histogram `i18n.format.date.duration_ms` (unit `ms`) — per-format wall-clock time
- Counter `i18n.format.cache_hit{formatter="date"}` — increments on cache reuse only
- In-memory state ring (`getDateMetricsState()`) for spec-conformance
- `resetDateMetricsState()` test-only helper to clear the ring

## 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 { formatDate } from "@suaveplan/i18n-formatter-date";
formatDate(new Date(2026, 0, 1), "en-GB", { preset: "short" });
// emits via your reader
```

## Usage: Advanced Example

```ts
import { describe, expect, it } from "@suaveplan/testing/runner";
import {
    clearDateFormatterCache,
    formatDate,
} from "@suaveplan/i18n-formatter-date";
import {
    getDateMetricsState,
    resetDateMetricsState,
} from "@suaveplan/i18n-formatter-date";

it("emits histogram + cache-hit counter", () => {
    clearDateFormatterCache();
    resetDateMetricsState();
    formatDate(new Date(2026, 0, 1), "en-GB", { preset: "short" });
    formatDate(new Date(2026, 0, 1), "en-GB", { preset: "short" });
    const state = getDateMetricsState();
    expect(state.durationSamples).toHaveLength(2);
    expect(state.cacheHitSamples).toHaveLength(1);
    expect(state.durationSamples[0]?.attributes).toMatchObject({
        locale: "en-GB",
        preset: "short",
    });
});
```

## API Reference

### `recordDateFormat(locale, preset, durationMs): void`

Records the duration histogram sample. Tags: `locale`, `preset`.
Called internally by `formatDate` / `formatDateRange`.

### `recordDateCacheHit(): void`

Increments the cache-hit counter with the constant tag
`{ formatter: "date" }`.

### `getDateMetricsState(): DateMetricsState`

Returns the in-memory ring of `durationSamples` and
`cacheHitSamples`. Used by spec-conformance tests.

### `resetDateMetricsState(): void`

Clears both ring buffers. Test-only helper.

## Implementation Notes

- The OTel meter is named `suaveplan.i18n.formatter.date`. Every
  formatter in the family has its own meter so OTel resource
  attributes can be tied per-package if a consumer chooses.
- When the caller omits `preset`, the histogram tag is `"custom"` so
  every sample carries a non-null preset attribute.
- Sample dashboard query (PromQL/Grafana over OTLP):
  `histogram_quantile(0.99, sum(rate(i18n_format_date_duration_ms_bucket[5m])) by (le, locale, preset))`
- The in-memory ring is intentionally unbounded — tests are short
  and call `resetDateMetricsState()` between cases. A long-running
  test loop should call reset to avoid memory growth.