feat(tracing): Emit app.vitals.start.screen on standalone app start (RN-691)

Set the `app.vitals.start.screen` attribute on the standalone `app.start`
transaction from the current route tracked by the tracing integration. Unlike
the non-standalone `ui.load` transaction (named after the screen, which Relay
backfills from), the standalone transaction is named `App Start`, so the screen
is set explicitly. Omitted when no route has been registered yet at capture time.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: antonis

packages/core/src/js/tracing/integrations/appStart.ts

@@ -31,7 +31,9 @@ import {
   UI_LOAD as UI_LOAD_OP,
 } from '../ops';
 import { SPAN_ORIGIN_AUTO_APP_START, SPAN_ORIGIN_MANUAL_APP_START } from '../origin';
+import { getCurrentReactNativeTracingIntegration } from '../reactnativetracing';
 import {
+  SEMANTIC_ATTRIBUTE_APP_VITALS_START_SCREEN,
   SEMANTIC_ATTRIBUTE_APP_VITALS_START_TYPE,
   SEMANTIC_ATTRIBUTE_APP_VITALS_START_VALUE,
   SEMANTIC_ATTRIBUTE_SENTRY_OP,
@@ -659,6 +661,15 @@ export const appStartIntegration = ({
       event.contexts.trace.data[SEMANTIC_ATTRIBUTE_APP_VITALS_START_VALUE] = appStartDurationMs;
       event.contexts.trace.data[SEMANTIC_ATTRIBUTE_APP_VITALS_START_TYPE] = appStart.type;
 
+      // Screen shown when app start completes. Unlike the non-standalone `ui.load` transaction
+      // (whose name is the screen, which Relay backfills from), the standalone transaction is named
+      // `App Start`, so we set the screen explicitly. Sourced from the current route tracked by the
+      // tracing integration; omitted when no route has been registered yet at capture time.
+      const screen = getCurrentReactNativeTracingIntegration()?.state.currentRoute;
+      if (screen) {
+        event.contexts.trace.data[SEMANTIC_ATTRIBUTE_APP_VITALS_START_SCREEN] = screen;
+      }
+
       // Minimal parent referencing the root transaction span, so the breakdown spans attach
       // directly under it (the helpers only read op/origin/span_id/trace_id/start_timestamp).
       // `data` is shared with the root trace context so frame data lands on the root span.

packages/core/src/js/tracing/semanticAttributes.ts

@@ -22,3 +22,4 @@ export const SEMANTIC_ATTRIBUTE_NAVIGATION_ACTION_TYPE = 'navigation.action_type
 // App start vitals (Span V2 / EAP). Emitted on the standalone `app.start` transaction.
 export const SEMANTIC_ATTRIBUTE_APP_VITALS_START_VALUE = 'app.vitals.start.value';
 export const SEMANTIC_ATTRIBUTE_APP_VITALS_START_TYPE = 'app.vitals.start.type';
+export const SEMANTIC_ATTRIBUTE_APP_VITALS_START_SCREEN = 'app.vitals.start.screen';

packages/core/test/tracing/integrations/appStart.test.ts

@@ -36,7 +36,9 @@ import {
   setRootComponentCreationTimestampMs,
 } from '../../../src/js/tracing/integrations/appStart';
 import { SPAN_ORIGIN_AUTO_APP_START, SPAN_ORIGIN_MANUAL_APP_START } from '../../../src/js/tracing/origin';
+import * as ReactNativeTracing from '../../../src/js/tracing/reactnativetracing';
 import {
+  SEMANTIC_ATTRIBUTE_APP_VITALS_START_SCREEN,
   SEMANTIC_ATTRIBUTE_APP_VITALS_START_TYPE,
   SEMANTIC_ATTRIBUTE_APP_VITALS_START_VALUE,
 } from '../../../src/js/tracing/semanticAttributes';
@@ -147,6 +149,32 @@ describe('App Start Integration', () => {
       });
     });
 
+    it('sets app.vitals.start.screen from the current route', async () => {
+      const [timeOriginMilliseconds, appStartTimeMilliseconds] = mockAppStart({ cold: true });
+      const screenSpy = jest
+        .spyOn(ReactNativeTracing, 'getCurrentReactNativeTracingIntegration')
+        .mockReturnValue({ state: { currentRoute: 'HomeScreen' } } as ReturnType<
+          typeof ReactNativeTracing.getCurrentReactNativeTracingIntegration
+        >);
+
+      try {
+        const actualEvent = await captureStandAloneAppStart();
+        expect(actualEvent).toEqual(
+          expectEventWithStandaloneColdAppStart(actualEvent, { timeOriginMilliseconds, appStartTimeMilliseconds }),
+        );
+        expect(actualEvent?.contexts?.trace?.data?.[SEMANTIC_ATTRIBUTE_APP_VITALS_START_SCREEN]).toBe('HomeScreen');
+      } finally {
+        screenSpy.mockRestore();
+      }
+    });
+
+    it('omits app.vitals.start.screen when no route has been registered', async () => {
+      mockAppStart({ cold: true });
+
+      const actualEvent = await captureStandAloneAppStart();
+      expect(actualEvent?.contexts?.trace?.data).not.toHaveProperty(SEMANTIC_ATTRIBUTE_APP_VITALS_START_SCREEN);
+    });
+
     it('Does not add any spans or measurements when App Start Span is longer than threshold', async () => {
       set__DEV__(false);
       mockTooLongAppStart();