metrics: Randomly initialize CachedMetrics with histogram cache bypassed

Make CachedMetrics during initialization have 50% chance to have all
histogram samples bypass it's cache and immediately forward metrics to
RecordHistogram (which now uses CachingUmaRecorder).

The state of the bypass is reported with a synthetic field trial:
 -  AndroidCachedMetrics.Control
 -  AndroidCachedMetrics.HistogramsBypassCache

Bug: 1046181
Change-Id: I7fe1d9d4edf6ccf8a6b2dde0348324aaf797f032
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031443
Commit-Queue: bttk <bttk@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738384}

base/android/java/src/org/chromium/base/metrics/CachedMetrics.java

@@ -28,6 +28,12 @@ public class CachedMetrics {
         @GuardedBy("sMetrics")
         private static final List<CachedMetric> sMetrics = new ArrayList<CachedMetric>();
 
+        /**
+         * Randomly choose whether to always immediately invoke {@link RecordHistogram}. This value
+         * is recorded using a synthetic field trial.
+         */
+        protected static final boolean HISTOGRAMS_BYPASS_CACHE = Math.random() < 0.5;
+
         protected final String mName;
         protected boolean mCached;
 
@@ -59,6 +65,19 @@ public class CachedMetrics {
          */
         @GuardedBy("sMetrics")
         protected abstract void commitAndClear();
+
+        /**
+         * Returns {@code true} if histogram samples should be immediately recorded.
+         * <p>
+         * Handles two cases:
+         * <ul>
+         *     <li>Cache is not needed, because {@link LibraryLoader} is already initialized.
+         *     <li>Cache is disabled to validate caching present in {@link RecordHistogram}.
+         * </ul>
+         */
+        protected static boolean shouldHistogramBypassCache() {
+            return HISTOGRAMS_BYPASS_CACHE || LibraryLoader.getInstance().isInitialized();
+        }
     }
 
     /**
@@ -113,7 +132,7 @@ public class CachedMetrics {
 
         public void record(int sample) {
             synchronized (CachedMetric.sMetrics) {
-                if (LibraryLoader.getInstance().isInitialized()) {
+                if (shouldHistogramBypassCache()) {
                     recordWithNative(sample);
                 } else {
                     mSamples.add(sample);
@@ -153,7 +172,7 @@ public class CachedMetrics {
 
         public void record(int sample) {
             synchronized (CachedMetric.sMetrics) {
-                if (LibraryLoader.getInstance().isInitialized()) {
+                if (shouldHistogramBypassCache()) {
                     recordWithNative(sample);
                 } else {
                     mSamples.add(sample);
@@ -192,7 +211,7 @@ public class CachedMetrics {
 
         public void record(long sample) {
             synchronized (CachedMetric.sMetrics) {
-                if (LibraryLoader.getInstance().isInitialized()) {
+                if (shouldHistogramBypassCache()) {
                     recordWithNative(sample);
                 } else {
                     mSamples.add(sample);
@@ -249,7 +268,7 @@ public class CachedMetrics {
 
         public void record(boolean sample) {
             synchronized (CachedMetric.sMetrics) {
-                if (LibraryLoader.getInstance().isInitialized()) {
+                if (shouldHistogramBypassCache()) {
                     recordWithNative(sample);
                 } else {
                     mSamples.add(sample);
@@ -295,7 +314,7 @@ public class CachedMetrics {
 
         public void record(int sample) {
             synchronized (CachedMetric.sMetrics) {
-                if (LibraryLoader.getInstance().isInitialized()) {
+                if (shouldHistogramBypassCache()) {
                     recordWithNative(sample);
                 } else {
                     mSamples.add(sample);
@@ -385,4 +404,9 @@ public class CachedMetrics {
             }
         }
     }
+
+    /** Returns true if histograms are immediately recorded with {@link UmaRecorder}. */
+    public static boolean histogramsBypassCache() {
+        return CachedMetric.HISTOGRAMS_BYPASS_CACHE;
+    }
 }

chrome/android/java/src/org/chromium/chrome/browser/metrics/LaunchMetrics.java

@@ -95,6 +95,10 @@ public class LaunchMetrics {
         }
         sHomeScreenLaunches.clear();
 
+        String cachedMetricsGroup =
+                CachedMetrics.histogramsBypassCache() ? "HistogramsBypassCache" : "Control";
+        UmaSessionStats.registerSyntheticFieldTrial("AndroidCachedMetrics", cachedMetricsGroup);
+
         // Record generic cached events.
         CachedMetrics.commitCachedMetrics();
     }

chrome/android/java/src/org/chromium/chrome/browser/metrics/UmaSessionStats.java

@@ -10,6 +10,7 @@ import android.content.res.Configuration;
 import android.text.TextUtils;
 
 import org.chromium.base.ApplicationStatus;
+import org.chromium.base.Log;
 import org.chromium.base.annotations.CalledByNative;
 import org.chromium.base.annotations.NativeMethods;
 import org.chromium.base.library_loader.LibraryProcessType;
@@ -33,6 +34,7 @@ import org.chromium.content_public.browser.WebContents;
  * and the framework's MetricService.
  */
 public class UmaSessionStats {
+    private static final String TAG = "UmaSessionStats";
     private static final String SAMSUNG_MULTWINDOW_PACKAGE = "com.sec.feature.multiwindow";
 
     private static long sNativeUmaSessionStats;
@@ -213,6 +215,7 @@ public class UmaSessionStats {
     }
 
     public static void registerSyntheticFieldTrial(String trialName, String groupName) {
+        Log.d(TAG, "registerSyntheticFieldTrial(%s, %s)", trialName, groupName);
         assert isMetricsServiceAvailable();
         UmaSessionStatsJni.get().registerSyntheticFieldTrial(trialName, groupName);
     }