Avoid NaN in Commit Offset Failure Rate OffsetCommitServiceMetrics by… (#292)

* Avoid NaN in Commit Offset Failure Rate OffsetCommitServiceMetrics by initializing with a zero only when instantiating.
Signed-off-by: Andrew Choi <[email protected]>
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Author: Andrew Choi

src/main/java/com/linkedin/xinfra/monitor/services/metrics/OffsetCommitServiceMetrics.java

@@ -51,6 +51,8 @@ public OffsetCommitServiceMetrics(final Metrics metrics, final Map<String, Strin
         new Total());
 
     _offsetCommitFailSensor = metrics.sensor(FAILURE_SENSOR_NAME);
+    /* NaN will persist as long as no record is submitted to the failure sensor.
+       we'll continue with NaN for now since we'd rather that the Sensor itself is a true and unaltered record of what values it recorded. */
     _offsetCommitFailSensor.add(new MetricName(FAILURE_RATE_METRIC, METRIC_GROUP_NAME,
         "The failure rate of group coordinator accepting consumer offset commit requests.", tags), new Avg());
     _offsetCommitFailSensor.add(new MetricName(FAILURE_METRIC_TOTAL, METRIC_GROUP_NAME,