Bug: DropwizardMeterRegistry does not remove Gauges created by LongTaskTimer

[hendryluk]

Steps to reproduce:

1. Create a LongTaskTime onto a DropwizardMeterRegistry (e.g. JMX)

   MeterRegistry registry = newJmxMeterRegistry(JmxConfig.DEFAULT, Clock.SYSTEM);
   LongTaskTimer timer = registry.more().longTaskTimer("foo"); 
   timer.record(() => {});

   In JMX, this creates 3 gauge mbeans: foo.statistic.active, foo.statistic.max, and foo.statistic.duration.
2. Remove the timer

   registry.remove(timer);
   timer.close();

Expected:

It should remove all those 3 gauges from the underlying registry (i.e. JMX in this case).

Actual:

All the 3 gauges associated with the LongTimerMetric remain in JMX

Cause:

DropwizardMeterRegistry#newLongTaskTimer creates 3 gauges (Active, Duration, and Max). However it never gets removed during DropwizardMeterRegistry#onMeterRemoved

Fix

In DropwizardMeterRegistry#onMeterRemoved, add the following:

if (meter instanceof LongTaskTimer) {
   for (Statistic statistic: Statistic.values()) {
      registry.remove(hierarchicalName(meter.getId().withTag(statistic)));
   }
}

[shakuzen]

Thanks for the detailed report. It sounds like this should also be a problem for removing custom Meter implementations and Timer/DistributionSummary with histogram gauges configured.

Would you be interested and willing to submit a pull request for the fix to the 1.7.x branch?