Add volatile to the Log4j 2 OpenTelemetry appender (#9749)

Author: jeanbisutti

instrumentation/log4j/log4j-appender-2.17/library/src/main/java/io/opentelemetry/instrumentation/log4j/appender/v2_17/OpenTelemetryAppender.java

@@ -46,7 +46,7 @@ public class OpenTelemetryAppender extends AbstractAppender {
   static final String PLUGIN_NAME = "OpenTelemetry";
 
   private final LogEventMapper<ReadOnlyStringMap> mapper;
-  private OpenTelemetry openTelemetry;
+  private volatile OpenTelemetry openTelemetry;
 
   /**
    * Installs the {@code openTelemetry} instance on any {@link OpenTelemetryAppender}s identified in