Bucketed metrics: Use separate object for double-checked locking

According to Findbugs:

  "This method performs synchronization an object that is an instance
  of a class from the java.util.concurrent package (or its subclasses).
  Instances of these classes have their own concurrency control
  mechanisms that are orthogonal to the synchronization provided by the
  Java keyword synchronized."

Instead of synchronizing on the instance of ConcurrentHashMap, use a
separate Object.

Change-Id: Ie2c670c2a8617c453f5aa4005f51370c9772c77a

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/BucketedCallback.java

@@ -38,6 +38,7 @@ abstract class BucketedCallback<V> implements BucketedMetric {
   private final V zero;
   private final Map<Object, ValueGauge> cells;
   protected volatile Runnable trigger;
+  private final Object lock = new Object();
 
   BucketedCallback(DropWizardMetricMaker metrics, MetricRegistry registry,
       String name, Class<V> valueType, Description desc, Field<?>... fields) {
@@ -94,7 +95,7 @@ abstract class BucketedCallback<V> implements BucketedMetric {
       return c;
     }
 
-    synchronized (cells) {
+    synchronized (lock) {
       c = cells.get(key);
       if (c == null) {
         c = new ValueGauge();

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/BucketedCounter.java

@@ -35,6 +35,7 @@ abstract class BucketedCounter implements BucketedMetric {
   protected final Field<?>[] fields;
   protected final CounterImpl total;
   private final Map<Object, CounterImpl> cells;
+  private final Object lock = new Object();
 
   BucketedCounter(DropWizardMetricMaker metrics,
       String name, Description desc, Field<?>... fields) {
@@ -69,7 +70,7 @@ abstract class BucketedCounter implements BucketedMetric {
       return c;
     }
 
-    synchronized (cells) {
+    synchronized (lock) {
       c = cells.get(key);
       if (c == null) {
         c = metrics.newCounterImpl(submetric(key), isRate);

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/BucketedHistogram.java

@@ -34,6 +34,7 @@ abstract class BucketedHistogram implements BucketedMetric {
   protected final Field<?>[] fields;
   protected final HistogramImpl total;
   private final Map<Object, HistogramImpl> cells;
+  private final Object lock = new Object();
 
   BucketedHistogram(DropWizardMetricMaker metrics, String name,
       Description desc, Field<?>... fields) {
@@ -67,7 +68,7 @@ abstract class BucketedHistogram implements BucketedMetric {
       return c;
     }
 
-    synchronized (cells) {
+    synchronized (lock) {
       c = cells.get(key);
       if (c == null) {
         c = metrics.newHistogramImpl(submetric(key));

gerrit-server/src/main/java/com/google/gerrit/metrics/dropwizard/BucketedTimer.java

@@ -34,6 +34,7 @@ abstract class BucketedTimer implements BucketedMetric {
   protected final Field<?>[] fields;
   protected final TimerImpl total;
   private final Map<Object, TimerImpl> cells;
+  private final Object lock = new Object();
 
   BucketedTimer(DropWizardMetricMaker metrics, String name,
       Description desc, Field<?>... fields) {
@@ -67,7 +68,7 @@ abstract class BucketedTimer implements BucketedMetric {
       return c;
     }
 
-    synchronized (cells) {
+    synchronized (lock) {
       c = cells.get(key);
       if (c == null) {
         c = metrics.newTimerImpl(submetric(key));