Add logic to flush metrics for a given app state and add unit test

Author: tejasd

firebase-perf/src/main/java/com/google/firebase/perf/session/gauges/GaugeManager.java

@@ -100,12 +100,17 @@ public void initializeGaugeMetadataManager(Context appContext) {
 
   @Override
   public void onUpdateAppState(ApplicationProcessState applicationProcessState) {
-    this.applicationProcessState = applicationProcessState;
-
+    // Update the app state and return.
     if (session == null || !session.isVerbose()) {
+      this.applicationProcessState = applicationProcessState;
       return;
     }
 
+    // Log existing gauges to the current app state.
+    logGaugeMetrics();
+    // Update App State.
+    this.applicationProcessState = applicationProcessState;
+
     // If it's a verbose session, start collecting gauges for the new app state.
     startCollectingGauges(this.applicationProcessState, session.getTimer());
   }

firebase-perf/src/test/java/com/google/firebase/perf/session/gauges/GaugeManagerTest.java

@@ -357,6 +357,42 @@ public void testGaugeCounterStartsAJobToConsumeTheGeneratedMetrics() {
     assertThat(recordedGaugeMetric.getSessionId()).isEqualTo(testSessionId(1));
   }
 
+  @Test
+  public void testUpdateAppStateFlushesMetricsInTheCurrentAppState() {
+    PerfSession fakeSession = createTestSession(1);
+    fakeSession.setGaugeAndEventCollectionEnabled(true);
+    testGaugeManager.setApplicationProcessState(ApplicationProcessState.FOREGROUND);
+    testGaugeManager.startCollectingGauges(fakeSession);
+    GaugeCounter.INSTANCE.setGaugeManager(testGaugeManager);
+
+    // There's no job to log the gauges.
+    assertThat(fakeScheduledExecutorService.isEmpty()).isTrue();
+
+    // Generate metrics that don't exceed the GaugeCounter.MAX_COUNT.
+    generateMetricsAndIncrementCounter(10);
+
+    // There's still no job to log the gauges.
+    assertThat(fakeScheduledExecutorService.isEmpty()).isTrue();
+
+    testGaugeManager.onUpdateAppState(ApplicationProcessState.BACKGROUND);
+
+    assertThat(fakeScheduledExecutorService.isEmpty()).isFalse();
+    assertThat(fakeScheduledExecutorService.getDelayToNextTask(TimeUnit.MILLISECONDS))
+        .isEqualTo(TIME_TO_WAIT_BEFORE_FLUSHING_GAUGES_QUEUE_MS);
+
+    fakeScheduledExecutorService.simulateSleepExecutingAtMostOneTask();
+    GaugeMetric recordedGaugeMetric =
+        getLastRecordedGaugeMetric(ApplicationProcessState.FOREGROUND, 1);
+
+    // It flushes all metrics in the ConcurrentLinkedQueues.
+    int recordedGaugeMetricsCount =
+        recordedGaugeMetric.getAndroidMemoryReadingsCount()
+            + recordedGaugeMetric.getCpuMetricReadingsCount();
+    assertThat(recordedGaugeMetricsCount).isEqualTo(10);
+
+    assertThat(recordedGaugeMetric.getSessionId()).isEqualTo(testSessionId(1));
+  }
+
   @Test
   public void testStopCollectingGaugesStopsCollectingAllGaugeMetrics() {
     PerfSession fakeSession = createTestSession(1);