Add merge to Distribution

stats/src/main/java/com/facebook/airlift/stats/Distribution.java

@@ -57,7 +57,11 @@ public synchronized void add(long value, long count)
         digest.add(value, count);
         total.add(value * count);
     }
-
+    public synchronized void merge(Distribution distribution)
+    {
+        total.merge(distribution.total);
+        digest.merge(distribution.digest);
+    }
     @Managed
     public synchronized double getMaxError()
     {

stats/src/test/java/com/facebook/airlift/stats/TestDistribution.java

@@ -0,0 +1,39 @@
+package com.facebook.airlift.stats;
+
+import org.testng.annotations.Test;
+
+import static org.testng.Assert.assertEquals;
+
+public class TestDistribution
+{
+    @Test
+    public void testMerge()
+    {
+        Distribution distribution1 = new Distribution();
+        Distribution distribution2 = new Distribution();
+
+        for (int i = 0; i < 10; i++) {
+            distribution1.add(i);
+        }
+
+        for (int i = 10; i < 20; i++) {
+            distribution2.add(i);
+        }
+
+        distribution1.merge(distribution2);
+
+        assertEquals(distribution1.getP50(), 10);
+        assertEquals(distribution1.getMin(), 0);
+        assertEquals(distribution1.getMax(), 19);
+        assertEquals(distribution1.getCount(), 20.0);
+    }
+
+    @Test(expectedExceptions = IllegalArgumentException.class, expectedExceptionsMessageRegExp = "Expected decayCounter to have alpha.*")
+    public void testMergeWithIncompatibleAlphaFails()
+    {
+        Distribution distribution1 = new Distribution(0.5);
+        Distribution distribution2 = new Distribution(0.7);
+
+        distribution1.merge(distribution2);
+    }
+}