test: Add custom reporter for test metrics (#18960)

packages/testing/playwright/utils/performance-helper.ts

@@ -1,4 +1,4 @@
-import type { Page } from '@playwright/test';
+import type { Page, TestInfo } from '@playwright/test';
 
 export async function measurePerformance(
 	page: Page,
@@ -28,3 +28,21 @@ export async function getAllPerformanceMetrics(page: Page) {
 		return metrics;
 	});
 }
+
+/**
+ * Attach a performance metric for collection by the metrics reporter
+ * @param testInfo - The Playwright TestInfo object
+ * @param metricName - Name of the metric (will be prefixed with 'metric:')
+ * @param value - The numeric value to track
+ * @param unit - The unit of measurement (e.g., 'ms', 'bytes', 'count')
+ */
+export async function attachMetric(
+	testInfo: TestInfo,
+	metricName: string,
+	value: number,
+	unit?: string,
+): Promise<void> {
+	await testInfo.attach(`metric:${metricName}`, {
+		body: JSON.stringify({ value, unit }),
+	});
+}