Don't report empty histograms to FastForward (#140)

*Rationale*

If histograms don't have any reported values, or have not been used
in a long time, the snapshot is empty and the histogram metrics
will not have a reasonable default value. All stats (min, max, median, etc) would
incorrectly be reported as zero

*Performance impact*

The performance impact should be minimal, since it only adds one additional
if-check when reporting histograms. Snapshot.size() is a fast method, simply
returning the length of its internal array.

For cases where snapshots are empty - i.e. unused histograms we should see
a performance improvement since we can skip emitting the data points to FastForward
as well as skipping the creation of the metric ids.

ffwd-reporter/src/main/java/com/spotify/metrics/ffwd/FastForwardReporter.java

@@ -286,14 +286,19 @@ private void reportCounter(MetricId key, Counting value) {
     }
 
     private void reportHistogram(MetricId key, Histogram value) {
+        final Snapshot snapshot = value.getSnapshot();
+        if (snapshot.size() == 0) {
+            return;
+        }
+
         key = MetricId.join(prefix, key);
 
         final Metric m = FastForward
             .metric(key.getKey())
             .attributes(key.getTags())
             .attribute(METRIC_TYPE, "histogram");
 
-        reportHistogram(m, value.getSnapshot());
+        reportHistogram(m, snapshot);
     }
 
     private void reportMetered(MetricId key, Meter value) {
@@ -310,6 +315,11 @@ private void reportMetered(MetricId key, Meter value) {
     }
 
     private void reportTimer(MetricId key, Timer value) {
+        final Snapshot snapshot = value.getSnapshot();
+        if (snapshot.size() == 0) {
+            return;
+        }
+
         key = MetricId.join(prefix, key);
 
         final Metric m = FastForward
@@ -319,7 +329,7 @@ private void reportTimer(MetricId key, Timer value) {
             .attribute("unit", "ns");
 
         reportMetered(m, value);
-        reportHistogram(m, value.getSnapshot());
+        reportHistogram(m, snapshot);
     }
 
     private void reportDerivingMeter(MetricId key, DerivingMeter value) {

ffwd-reporter/src/test/java/com/spotify/metrics/ffwd/FastForwardReporterTest.java

@@ -245,4 +245,26 @@ public void testKeyValuePrefixAddedOnce() throws Exception {
 
         assertEquals(expectedKeys, actualKeys);
     }
+
+    @Test
+    public void shouldNotReportEmptyHistogramsAndTimers() throws Exception {
+        ArgumentCaptor<Metric> argumentCaptor = ArgumentCaptor.forClass(Metric.class);
+
+        doNothing().when(fastForward).send(argumentCaptor.capture());
+
+        MetricId name = MetricId.build("thename");
+
+        registry.histogram(name.tagged("histogram", "true"));
+        registry.timer(name.tagged("timer", "true"));
+
+        reporter.start();
+
+        executorService.tick(REPORTING_PERIOD + REPORTING_PERIOD / 3, TimeUnit.MILLISECONDS);
+        verify(fastForward, atLeastOnce()).send(any(Metric.class));
+
+        Set<String> actualKeys = argumentCaptor.getAllValues().stream().map(Metric::getKey)
+                .collect(Collectors.toSet());
+
+        assertEquals(new HashSet<>(Arrays.asList("test.hi")), actualKeys);
+    }
 }