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

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.
  *
@@ -114,6 +116,9 @@ export function getDefaultIntegrations(options: ReactNativeClientOptions): Integ
   const hasTracingEnabled = typeof options.tracesSampleRate === 'number' || typeof options.tracesSampler === 'function';
   if (hasTracingEnabled && options.enableAppStartTracking && options.enableNative) {
     integrations.push(appStartIntegration({ standalone: !!options._experiments?.enableStandaloneAppStartTracing }));
+    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 +42,18 @@ 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);
+  });
 });