feat(browser-utils): Add FCP instrumentation handler and export INP_ENTRY_MAP

Add `addFcpInstrumentationHandler` using the existing `onFCP` web-vitals
library integration, following the same pattern as the other metric handlers.
Export `INP_ENTRY_MAP` from inp.ts for reuse in the new web vital spans module.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: logaretm

packages/browser-utils/src/metrics/inp.ts

@@ -54,7 +54,7 @@ export function startTrackingINP(): () => void {
   return () => undefined;
 }
 
-const INP_ENTRY_MAP: Record<string, 'click' | 'hover' | 'drag' | 'press'> = {
+export const INP_ENTRY_MAP: Record<string, 'click' | 'hover' | 'drag' | 'press'> = {
   click: 'click',
   pointerdown: 'click',
   pointerup: 'click',

packages/browser-utils/src/metrics/instrument.ts

@@ -4,6 +4,7 @@ import { onCLS } from './web-vitals/getCLS';
 import { onINP } from './web-vitals/getINP';
 import { onLCP } from './web-vitals/getLCP';
 import { observe } from './web-vitals/lib/observe';
+import { onFCP } from './web-vitals/onFCP';
 import { onTTFB } from './web-vitals/onTTFB';
 
 type InstrumentHandlerTypePerformanceObserver =
@@ -16,7 +17,7 @@ type InstrumentHandlerTypePerformanceObserver =
   // fist-input is still needed for INP
   | 'first-input';
 
-type InstrumentHandlerTypeMetric = 'cls' | 'lcp' | 'ttfb' | 'inp';
+type InstrumentHandlerTypeMetric = 'cls' | 'lcp' | 'ttfb' | 'inp' | 'fcp';
 
 // We provide this here manually instead of relying on a global, as this is not available in non-browser environements
 // And we do not want to expose such types
@@ -114,6 +115,7 @@ let _previousCls: Metric | undefined;
 let _previousLcp: Metric | undefined;
 let _previousTtfb: Metric | undefined;
 let _previousInp: Metric | undefined;
+let _previousFcp: Metric | undefined;
 
 /**
  * Add a callback that will be triggered when a CLS metric is available.
@@ -164,6 +166,14 @@ export function addInpInstrumentationHandler(callback: InstrumentationHandlerCal
   return addMetricObserver('inp', callback, instrumentInp, _previousInp);
 }
 
+/**
+ * Add a callback that will be triggered when a FCP metric is available.
+ * Returns a cleanup callback which can be called to remove the instrumentation handler.
+ */
+export function addFcpInstrumentationHandler(callback: (data: { metric: Metric }) => void): CleanupHandlerCallback {
+  return addMetricObserver('fcp', callback, instrumentFcp, _previousFcp);
+}
+
 export function addPerformanceInstrumentationHandler(
   type: 'event',
   callback: (data: { entries: ((PerformanceEntry & { target?: unknown | null }) | PerformanceEventTiming)[] }) => void,
@@ -259,6 +269,15 @@ function instrumentInp(): void {
   });
 }
 
+function instrumentFcp(): StopListening {
+  return onFCP(metric => {
+    triggerHandlers('fcp', {
+      metric,
+    });
+    _previousFcp = metric;
+  });
+}
+
 function addMetricObserver(
   type: InstrumentHandlerTypeMetric,
   callback: InstrumentHandlerCallback,