skills/docs/localization/i18n/i18n-formatter-display-name/src/observability/observability.md

DisplayName observability

(no submodules)

Purpose

The DisplayName 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 instruments match the canonical Wave-3 formatter contract — every sibling formatter (number, relative-time, list, date, currency, collator, segmenter) emits the same two metric names so dashboards and alerts compose cleanly across the family.

Features

• Histogram i18n.format.display_name.duration_ms (unit ms) — per-format wall-clock time
• Counter i18n.format.cache_hit{formatter="display_name"} — increments on cache reuse only
• In-memory state ring (getDisplayNameMetricsState()) for spec-conformance assertions
• resetDisplayNameMetricsState() test-only helper to clear the ring between cases

Usage: Basic Example

import { metrics } from "@opentelemetry/api";
import { MeterProvider } from "@opentelemetry/sdk-metrics";
// import your MetricReader / OTLP exporter ...

const provider = new MeterProvider({ readers: [/* reader */] });
metrics.setGlobalMeterProvider(provider);

// Subsequent formatDisplayName calls now emit through your reader.
import { formatDisplayName } from "@suaveplan/i18n-formatter-display-name";
formatDisplayName("fr", "en-GB", { type: "language" });

Usage: Advanced Example

import { describe, expect, it } from "@suaveplan/testing/runner";
import {
    clearDisplayNameCache,
    formatDisplayName,
} from "@suaveplan/i18n-formatter-display-name";
import {
    getDisplayNameMetricsState,
    resetDisplayNameMetricsState,
} from "@suaveplan/i18n-formatter-display-name";

it("emits histogram + cache-hit counter", () => {
    clearDisplayNameCache();
    resetDisplayNameMetricsState();
    formatDisplayName("fr", "en-GB", { type: "language" });
    formatDisplayName("fr", "en-GB", { type: "language" });
    const state = getDisplayNameMetricsState();
    expect(state.durationSamples).toHaveLength(2);
    expect(state.cacheHitSamples).toHaveLength(1);
});

API Reference

recordDisplayNameFormat(locale, type, style, durationMs): void

Records the duration histogram sample. Tags: locale, type, style. Called internally by formatDisplayName; consumers do not need to invoke it directly.

recordDisplayNameCacheHit(): void

Increments the cache-hit counter with the constant tag { formatter: "display_name" }. Called internally on a cache hit.

getDisplayNameMetricsState(): DisplayNameMetricsState

Returns the in-memory ring of durationSamples and cacheHitSamples. Used by spec-conformance tests to assert exact attribute payloads.

resetDisplayNameMetricsState(): void

Clears both ring buffers. Test-only helper; call before the under-test code to assert exactly the samples your scenario generates.

Implementation Notes

• The OTel meter is named suaveplan.i18n.formatter.display_name. Every formatter in the family has its own meter so OTel resource attributes can be tied per-package if a consumer chooses.
• Sample dashboard query (PromQL/Grafana over OTLP): histogram_quantile(0.99, sum(rate(i18n_format_display_name_duration_ms_bucket[5m])) by (le, locale))
• The in-memory ring is intentionally unbounded — tests are short and call resetDisplayNameMetricsState() between cases. A long-running test loop should call reset to avoid memory growth.
• style defaults to "long" in the histogram tag when the caller omits it; type is always required so it is never defaulted.