Intermittent test failure in CustomQuotaCallbackTest

Exception stack trace:

java.lang.NullPointerException at org.apache.kafka.common.metrics.stats.SampledStat.purgeObsoleteSamples(SampledStat.java:104) at org.apache.kafka.common.metrics.stats.SampledStat.measure(SampledStat.java:74) at org.apache.kafka.common.metrics.KafkaMetric.metricValue(KafkaMetric.java:68) at kafka.api.QuotaTestClients$.metricValue(BaseQuotaTest.scala:163) at kafka.api.QuotaTestClients.produceUntilThrottled(BaseQuotaTest.scala:193) at kafka.api.CustomQuotaCallbackTest$GroupedUser.produceConsume(CustomQuotaCallbackTest.scala:272) at kafka.api.CustomQuotaCallbackTest.testCustomQuotaCallback(CustomQuotaCallbackTest.scala:146)