Create a copy of the collections to remove in MetricsTestUtil.

Signed-off-by: David Venable <dlv@amazon.com>

data-prepper-api/src/test/java/org/opensearch/dataprepper/metrics/MetricsTestUtil.java

@@ -6,20 +6,28 @@
 package org.opensearch.dataprepper.metrics;
 
 import io.micrometer.core.instrument.Measurement;
+import io.micrometer.core.instrument.Meter;
 import io.micrometer.core.instrument.MeterRegistry;
 import io.micrometer.core.instrument.Metrics;
 import io.micrometer.core.instrument.Statistic;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
 
+import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.stream.Collectors;
 import java.util.stream.StreamSupport;
 
 public class MetricsTestUtil {
 
     public static synchronized void initMetrics() {
-        Metrics.globalRegistry.getRegistries().forEach(meterRegistry -> Metrics.globalRegistry.remove(meterRegistry));
-        Metrics.globalRegistry.getMeters().forEach(meter -> Metrics.globalRegistry.remove(meter));
+        final Set<MeterRegistry> registries = new HashSet<>(Metrics.globalRegistry.getRegistries());
+        registries.forEach(meterRegistry -> Metrics.globalRegistry.remove(meterRegistry));
+
+        final List<Meter> meters = new ArrayList<>(Metrics.globalRegistry.getMeters());
+        meters.forEach(meter -> Metrics.globalRegistry.remove(meter));
+
         Metrics.addRegistry(new SimpleMeterRegistry());
     }