feat(tracing): Track standalone app start adoption (#6427)

* feat(core): Track GlobalErrorBoundary adoption

Register a no-op `GlobalErrorBoundary` integration when the
component mounts so the name flows through to
`event.sdk.integrations` — the same channel used for feature-
adoption signals like `MobileFeedback` and `AppStart`.

Also introduces a shared `registerFeatureMarker` helper.
Subsequent markers for other opt-in features
(NavigationContainer, ExpoRouter error boundary, AppLoaded, ...)
will use this helper — see #6415.

Refs: #6415

* feat(tracing): Track standalone app start adoption

When `_experiments.enableStandaloneAppStartTracing` is enabled,
also add a no-op `StandaloneAppStart` integration to the default
integrations list so the name flows through to
`event.sdk.integrations`.

Refs: #6415

* fix(tracing): Fire StandaloneAppStart marker regardless of tracing config

The marker is meant as an adoption signal for the
`enableStandaloneAppStartTracing` experiment flag itself, so it
must reach errors-only projects and configs where tracing /
`enableAppStartTracking` / `enableNative` are off.

Reported by Cursor Bugbot on PR #6427.

Author: antonis

packages/core/src/js/integrations/default.ts

@@ -46,6 +46,8 @@ import {
   viewHierarchyIntegration,
 } from './exports';
 
+const STANDALONE_APP_START_INTEGRATION_NAME = 'StandaloneAppStart';
+
 /**
  * Returns the default ReactNative integrations based on the current environment.
  *
@@ -115,6 +117,12 @@ export function getDefaultIntegrations(options: ReactNativeClientOptions): Integ
   if (hasTracingEnabled && options.enableAppStartTracking && options.enableNative) {
     integrations.push(appStartIntegration({ standalone: !!options._experiments?.enableStandaloneAppStartTracing }));
   }
+  // Registered outside the tracing/appStart/native gate: this is an adoption
+  // marker for the experiment flag itself, so it must reach errors-only
+  // projects and any config where tracing is disabled.
+  if (options._experiments?.enableStandaloneAppStartTracing) {
+    integrations.push({ name: STANDALONE_APP_START_INTEGRATION_NAME });
+  }
   const nativeFramesIntegrationInstance = createNativeFramesIntegrations(
     hasTracingEnabled && options.enableNativeFramesTracking && options.enableNative,
   );

packages/core/test/integrations/defaultAppStart.test.ts

@@ -42,4 +42,30 @@ describe('getDefaultIntegrations - standalone app start wiring', () => {
 
     expect(appStartIntegration).toHaveBeenCalledWith({ standalone: false });
   });
+
+  it('includes the StandaloneAppStart marker when the experiment flag is enabled', () => {
+    const integrations = getDefaultIntegrations(
+      createOptions({ _experiments: { enableStandaloneAppStartTracing: true } }),
+    );
+
+    expect(integrations.some(i => i.name === 'StandaloneAppStart')).toBe(true);
+  });
+
+  it('does not include the StandaloneAppStart marker when the experiment flag is off', () => {
+    const integrations = getDefaultIntegrations(createOptions({}));
+
+    expect(integrations.some(i => i.name === 'StandaloneAppStart')).toBe(false);
+  });
+
+  it.each([
+    ['tracing disabled', { tracesSampleRate: undefined }],
+    ['app start tracking off', { enableAppStartTracking: false }],
+    ['native off', { enableNative: false }],
+  ])('includes the StandaloneAppStart marker when the flag is on and %s', (_label, overrides) => {
+    const integrations = getDefaultIntegrations(
+      createOptions({ ...overrides, _experiments: { enableStandaloneAppStartTracing: true } }),
+    );
+
+    expect(integrations.some(i => i.name === 'StandaloneAppStart')).toBe(true);
+  });
 });