🛑Disable all perf observers in inabox. (#28125)

* disable most observers inabox

* move disable to registerPerformanceObserver

src/service/performance-impl.js

@@ -289,6 +289,14 @@ export class Performance {
    * See https://github.com/WICG/paint-timing
    */
   registerPerformanceObserver_() {
+    // Turn off performanceObserver derived metrics for inabox as there
+    // will never be a viewer to report to.
+    // TODO(ccordry): we are still doing some other unnecessary measurements for
+    // the inabox case, but would need a larger refactor.
+    if (getMode(this.win).runtime === 'inabox') {
+      return;
+    }
+
     // Chromium doesn't implement the buffered flag for PerformanceObserver.
     // That means we need to read existing entries and maintain state
     // as to whether we have reported a value yet, since in the future it may

test/unit/test-performance.js

@@ -1351,4 +1351,29 @@ describes.realWin('PeformanceObserver metrics', {amp: true}, (env) => {
       ]);
     });
   });
+
+  describe('inabox environment', () => {
+    let PerformanceObserverConstructorStub;
+
+    beforeEach(() => {
+      PerformanceObserverConstructorStub = env.sandbox.stub(
+        env.win,
+        'PerformanceObserver'
+      );
+    });
+
+    it('disables many observers', () => {
+      PerformanceObserverConstructorStub.supportedEntryTypes = [
+        'navigation',
+        'largest-contentful-paint',
+        'first-input',
+        'layout-shift',
+      ];
+      installPerformanceService(env.win);
+      env.win.__AMP_MODE.runtime = 'inabox';
+      Services.performanceFor(env.win);
+      // Each supported entryType currently leads to creation of new observer.
+      expect(PerformanceObserverConstructorStub).not.to.be.called;
+    });
+  });
 });