Prevent NaN values when calculating mean

metrics-core/src/main/java/com/codahale/metrics/ExponentiallyDecayingReservoir.java

@@ -174,6 +174,9 @@ private void rescale(long now, long next) {
                     for (Double key : keys) {
                         final WeightedSample sample = values.remove(key);
                         final WeightedSample newSample = new WeightedSample(sample.value, sample.weight * scalingFactor);
+                        if (Double.compare(newSample.weight, 0) == 0) {
+                            continue;
+                        }
                         values.put(key * scalingFactor, newSample);
                     }
                 }

metrics-core/src/test/java/com/codahale/metrics/ExponentiallyDecayingReservoirTest.java

@@ -1,5 +1,6 @@
 package com.codahale.metrics;
 
+import com.codahale.metrics.Timer.Context;
 import org.junit.Test;
 
 import java.util.concurrent.TimeUnit;
@@ -140,6 +141,22 @@ public void emptyReservoirSnapshot_shouldReturnZeroForAllValues() {
         assertThat(snapshot.size()).isEqualTo(0);
     }
 
+    @Test
+    public void removeZeroWeightsInSamplesToPreventNaNInMeanValues() {
+        final ManualClock clock = new ManualClock();
+        final ExponentiallyDecayingReservoir reservoir = new ExponentiallyDecayingReservoir(1028, 0.015, clock);
+        Timer timer = new Timer(reservoir, clock);
+
+        Context context = timer.time();
+        clock.addMillis(100);
+        context.stop();
+
+        for (int i = 1; i < 48; i++) {
+            clock.addHours(1);
+            assertThat(reservoir.getSnapshot().getMean()).isBetween(0.0, Double.MAX_VALUE);
+        }
+    }
+
     @Test
     public void multipleUpdatesAfterlongPeriodsOfInactivityShouldNotCorruptSamplingState() throws Exception {
         // This test illustrates the potential race condition in rescale that