better name and comments

Author: leotianlizhan

firebase-perf/dev-app/src/main/AndroidManifest.xml

@@ -39,7 +39,7 @@
             android:name="fragment_sampling_percentage"
             android:value="100.0" />
         <meta-data
-            android:name="experiment_as_ttid"
+            android:name="experiment_app_start_ttid"
             android:value="true" />
 
         <receiver

firebase-perf/src/main/java/com/google/firebase/perf/config/ConfigResolver.java

@@ -768,7 +768,7 @@ public float getFragmentSamplingRate() {
     return config.getDefault();
   }
 
-  /** Returns if _experiment_as_ttid should be captured. */
+  /** Returns if _experiment_app_start_ttid should be captured. */
   public boolean getIsExperimentTTIDEnabled() {
     // Order of precedence is:
     // 1. If the value exists in Android Manifest, return this value.

firebase-perf/src/main/java/com/google/firebase/perf/config/ConfigurationConstants.java

@@ -683,7 +683,7 @@ protected Boolean getDefault() {
 
     @Override
     protected String getRemoteConfigFlag() {
-      return "fpr_experiment_as_ttid";
+      return "fpr_experiment_app_start_ttid";
     }
 
     @Override
@@ -693,7 +693,7 @@ protected String getDeviceCacheFlag() {
 
     @Override
     protected String getMetadataFlag() {
-      return "experiment_as_ttid";
+      return "experiment_app_start_ttid";
     }
   }
 }

firebase-perf/src/main/java/com/google/firebase/perf/metrics/AppStartTrace.java

@@ -248,7 +248,8 @@ public synchronized void onActivityResumed(Activity activity) {
     }
 
     // Shadow-launch experiment of new app start time
-    if (configResolver.getIsExperimentTTIDEnabled()) {
+    final boolean isExperimentTTIDEnabled = configResolver.getIsExperimentTTIDEnabled();
+    if (isExperimentTTIDEnabled) {
       View rootView = activity.findViewById(android.R.id.content);
       FirstDrawDoneListener.registerForNextDraw(rootView, this::recordFirstDrawDone);
     }
@@ -272,12 +273,17 @@ public synchronized void onActivityResumed(Activity activity) {
 
     // Log the app start trace in a non-main thread.
     executorService.execute(this::logAppStartTrace);
+
+    if (!isExperimentTTIDEnabled && isRegisteredForLifecycleCallbacks) {
+      // After AppStart trace is logged, we can unregister this callback.
+      unregisterActivityLifecycleCallbacks();
+    }
   }
 
   private void logColdStart(Timer start, Timer end, PerfSession session) {
     TraceMetric.Builder metric =
         TraceMetric.newBuilder()
-            .setName("_experiment_as_ttid")
+            .setName("_experiment_app_start_ttid")
             .setClientStartTimeUs(start.getMicros())
             .setDurationUs(start.getDurationMicros(end));
 

firebase-perf/src/test/java/com/google/firebase/perf/metrics/AppStartTraceTest.java

@@ -283,15 +283,16 @@ public void testFirebasePerfProviderOnAttachInfo_initializesGaugeCollection() {
   @Test
   @Config(sdk = 26)
   public void timeToInitialDisplay_isLogged() {
+    // Test setup
     when(clock.getTime()).thenCallRealMethod(); // Use robolectric shadows to manipulate time
-    long appStartTime = TimeUnit.MILLISECONDS.toMicros(Process.getStartElapsedRealtime());
     View testView = new View(ApplicationProvider.getApplicationContext());
     when(activity1.findViewById(android.R.id.content)).thenReturn(testView);
     when(configResolver.getIsExperimentTTIDEnabled()).thenReturn(true);
     FakeScheduledExecutorService fakeExecutorService = new FakeScheduledExecutorService();
     AppStartTrace trace =
         new AppStartTrace(transportManager, clock, configResolver, fakeExecutorService);
 
+    // Simulate resume and manually stepping time forward
     ShadowSystemClock.advanceBy(Duration.ofMillis(1000));
     long resumeTime = TimeUnit.NANOSECONDS.toMicros(SystemClock.elapsedRealtimeNanos());
     trace.onActivityCreated(activity1, bundle);
@@ -300,6 +301,8 @@ public void timeToInitialDisplay_isLogged() {
     fakeExecutorService.runAll();
     verify(transportManager, times(1))
         .log(isA(TraceMetric.class), isA(ApplicationProcessState.class));
+
+    // Simulate draw and manually stepping time forward
     ShadowSystemClock.advanceBy(Duration.ofMillis(1000));
     long drawTime = TimeUnit.NANOSECONDS.toMicros(SystemClock.elapsedRealtimeNanos());
     testView.getViewTreeObserver().dispatchOnDraw();
@@ -308,8 +311,10 @@ public void timeToInitialDisplay_isLogged() {
     verify(transportManager, times(2))
         .log(traceArgumentCaptor.capture(), isA(ApplicationProcessState.class));
 
+    // Verify ttid trace is being logged
     TraceMetric ttid = traceArgumentCaptor.getValue();
-    assertThat(ttid.getName()).isEqualTo("_experiment_as_ttid");
+    long appStartTime = TimeUnit.MILLISECONDS.toMicros(Process.getStartElapsedRealtime());
+    assertThat(ttid.getName()).isEqualTo("_experiment_app_start_ttid");
     assertThat(ttid.getDurationUs()).isNotEqualTo(resumeTime - appStartTime);
     assertThat(ttid.getDurationUs()).isEqualTo(drawTime - appStartTime);
   }