Convert timers from nanos to seconds

Timer quantiles are converted from nanos to seconds when building summaries.

Closes #49

Author: eperott

common/src/main/java/com/zegelin/cassandra/exporter/CassandraMetricsUtilities.java

@@ -67,11 +67,13 @@ public long getCount() {
             @Override
             public Iterable<Interval> getIntervals() {
                 /*
-                  Cassandra's JmxTimerMBean converts the percentile values to microseconds,
-                  which differs to the values returned by Sampling.getSnapshot() (which are in nanoseconds).
+                  Cassandra's JmxTimerMBean converts the percentile values to a parameterised duration unit,
+                  (currently the only usage is microseconds), which differs to the values returned by
+                  Sampling.getSnapshot() (which are always in nanoseconds).
 
-                  We pay the penalty when running out-of-process to convert from nanoseconds->microseconds->nanoseconds
-                  (and eventually to seconds)!
+                  The conversion from nanoseconds -> seconds happens at a later stage. To keep things simple, we pay the
+                  penalty when running out-of-process to convert from nanoseconds-><duration-unit>->nanoseconds
+                  and then eventually to seconds!
                  */
                 final TimeUnit durationUnit = TimeUnit.valueOf(timer.getDurationUnit().toUpperCase(Locale.US));
                 final float durationFactor = durationUnit.toNanos(1L);

common/src/main/java/com/zegelin/cassandra/exporter/FactoriesSupplier.java

@@ -407,7 +407,9 @@ private static FactoryBuilder.CollectorConstructor timerAsSummaryCollectorConstr
         return (name, help, labels, mBean) -> {
             final NamedObject<SamplingCounting> samplingCountingNamedObject = CassandraMetricsUtilities.jmxTimerMBeanAsSamplingCounting(mBean);
 
-            return new FunctionalMetricFamilyCollector<>(name, help, ImmutableMap.of(labels, samplingCountingNamedObject), samplingAndCountingAsSummary(MetricValueConversionFunctions::microsecondsToSeconds));
+            return new FunctionalMetricFamilyCollector<>(name, help, ImmutableMap.of(labels, samplingCountingNamedObject),
+                    samplingAndCountingAsSummary(MetricValueConversionFunctions::nanosecondsToSeconds));
+
         };
     }