For long task timers removes entries from Dropwizard registry

fixes gh-2924

Author: marcingrzejszczak

micrometer-core/src/main/java/io/micrometer/core/instrument/dropwizard/DropwizardMeterRegistry.java

@@ -71,6 +71,11 @@ public DropwizardMeterRegistry(DropwizardConfig config, MetricRegistry registry,
 
     private void onMeterRemoved(Meter meter) {
         registry.remove(hierarchicalName(meter.getId()));
+        if (meter instanceof LongTaskTimer) {
+            for (Statistic statistic : Statistic.values()) {
+                registry.remove(hierarchicalName(meter.getId().withTag(statistic)));
+            }
+        }
     }
 
     public MetricRegistry getDropwizardRegistry() {

micrometer-core/src/test/java/io/micrometer/core/instrument/dropwizard/DropwizardMeterRegistryTest.java

@@ -111,4 +111,13 @@ void removeShouldWork() {
         assertThat(registry.getDropwizardRegistry().getMeters()).isEmpty();
     }
 
+    @Issue("#2924")
+    @Test
+    void removeShouldWorkForLongTaskTimer() {
+        LongTaskTimer timer = LongTaskTimer.builder("foo").register(registry);
+        assertThat(registry.getDropwizardRegistry().getGauges()).hasSize(3);
+        registry.remove(timer);
+        assertThat(registry.getDropwizardRegistry().getGauges()).isEmpty();
+    }
+
 }