Add missing GaugeCounter increment. (#6966)

As part of looking into refactoring `GaugeCounter` I noticed I'd missed
this increment.

Author: tejasd

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

@@ -186,6 +186,7 @@ private synchronized void scheduleCpuMetricCollectionOnce(Timer referenceTime) {
                 CpuMetricReading currCpuReading = syncCollectCpuMetric(referenceTime);
                 if (currCpuReading != null) {
                   cpuMetricReadings.add(currCpuReading);
+                  GaugeCounter.INSTANCE.incrementCounter();
                 }
               },
               /* initialDelay */ 0,

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

@@ -149,6 +149,7 @@ private synchronized void scheduleMemoryMetricCollectionOnce(Timer referenceTime
                 AndroidMemoryReading memoryReading = syncCollectMemoryMetric(referenceTime);
                 if (memoryReading != null) {
                   memoryMetricReadings.add(memoryReading);
+                  GaugeCounter.INSTANCE.incrementCounter();
                 }
               },
               /* initialDelay */ 0,

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

@@ -335,7 +335,7 @@ public void testGaugeCounterStartsAJobToConsumeTheGeneratedMetrics() throws Inte
     PerfSession fakeSession = createTestSession(1);
     testGaugeManager.setApplicationProcessState(ApplicationProcessState.FOREGROUND);
     testGaugeManager.startCollectingGauges(fakeSession);
-    GaugeCounter.INSTANCE.setGaugeManager(testGaugeManager);
+    GaugeCounter.setGaugeManager(testGaugeManager);
 
     // There's no job to log the gauges.
     assertThat(fakeScheduledExecutorService.isEmpty()).isTrue();
@@ -377,7 +377,7 @@ public void testUpdateAppStateHandlesMultipleAppStates() {
     fakeSession.setGaugeAndEventCollectionEnabled(true);
     testGaugeManager.setApplicationProcessState(ApplicationProcessState.FOREGROUND);
     testGaugeManager.startCollectingGauges(fakeSession);
-    GaugeCounter.INSTANCE.setGaugeManager(testGaugeManager);
+    GaugeCounter.setGaugeManager(testGaugeManager);
 
     // Generate metrics that don't exceed the GaugeCounter.MAX_COUNT.
     generateMetricsAndIncrementCounter(10);