Makes CompactionExecutorsMetrics.registry volatile (#5379)

This variable is accessed by multiple thread outside of sync block and
is not final, so made if volatile to ensure changes are seen.

Author: keith-turner

server/tserver/src/main/java/org/apache/accumulo/tserver/metrics/CompactionExecutorsMetrics.java

@@ -47,7 +47,7 @@ public class CompactionExecutorsMetrics implements MetricsProducer {
   private volatile List<CeMetrics> ceMetricsList = List.of();
   private final Map<CompactionExecutorId,CeMetrics> ceMetricsMap = new HashMap<>();
   private final Map<CompactionExecutorId,ExMetrics> exCeMetricsMap = new HashMap<>();
-  private MeterRegistry registry = null;
+  private volatile MeterRegistry registry = null;
 
   // public so it can be closed by outside callers
   public class CeMetrics implements AutoCloseable {