Prevent NaN values when calculating mean

implementation/src/main/java/io/smallrye/metrics/app/ExponentiallyDecayingReservoir.java

@@ -207,6 +207,9 @@ private void rescale(long now, long next) {
                         final WeightedSnapshot.WeightedSample sample = values.remove(key);
                         final WeightedSnapshot.WeightedSample newSample = new WeightedSnapshot.WeightedSample(sample.value,
                                 sample.weight * scalingFactor);
+                        if (Double.compare(newSample.weight, 0) == 0) {
+                            continue;
+                        }
                         values.put(key * scalingFactor, newSample);
                     }
                 }

implementation/src/test/java/io/smallrye/metrics/histogram/ExponentiallyWeightedReservoirTest.java

@@ -0,0 +1,29 @@
+package io.smallrye.metrics.histogram;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.CoreMatchers.not;
+
+import org.eclipse.microprofile.metrics.Histogram;
+import org.junit.Assert;
+import org.junit.Test;
+
+import io.smallrye.metrics.app.ExponentiallyDecayingReservoir;
+import io.smallrye.metrics.app.HistogramImpl;
+
+public class ExponentiallyWeightedReservoirTest {
+
+    @Test
+    public void removeZeroWeightsInSamplesToPreventNaNInMeanValues() {
+        final TestingClock clock = new TestingClock();
+        final ExponentiallyDecayingReservoir reservoir = new ExponentiallyDecayingReservoir(1028, 0.015, clock);
+        Histogram histogram = new HistogramImpl(reservoir);
+
+        histogram.update(100);
+
+        for (int i = 1; i < 48; i++) {
+            clock.addHours(1);
+            Assert.assertThat(reservoir.getSnapshot().getMean(), not(equalTo(Double.NaN)));
+        }
+    }
+
+}

implementation/src/test/java/io/smallrye/metrics/histogram/TestingClock.java

@@ -0,0 +1,46 @@
+package io.smallrye.metrics.histogram;
+
+import java.util.concurrent.TimeUnit;
+
+import io.smallrye.metrics.app.Clock;
+
+public class TestingClock extends Clock {
+    private final long initialTicksInNanos;
+    long ticksInNanos;
+
+    public TestingClock(long initialTicksInNanos) {
+        this.initialTicksInNanos = initialTicksInNanos;
+        this.ticksInNanos = initialTicksInNanos;
+    }
+
+    public TestingClock() {
+        this(0L);
+    }
+
+    public synchronized void addNanos(long nanos) {
+        ticksInNanos += nanos;
+    }
+
+    public synchronized void addSeconds(long seconds) {
+        ticksInNanos += TimeUnit.SECONDS.toNanos(seconds);
+    }
+
+    public synchronized void addMillis(long millis) {
+        ticksInNanos += TimeUnit.MILLISECONDS.toNanos(millis);
+    }
+
+    public synchronized void addHours(long hours) {
+        ticksInNanos += TimeUnit.HOURS.toNanos(hours);
+    }
+
+    @Override
+    public synchronized long getTick() {
+        return ticksInNanos;
+    }
+
+    @Override
+    public synchronized long getTime() {
+        return TimeUnit.NANOSECONDS.toMillis(ticksInNanos - initialTicksInNanos);
+    }
+
+}