Add volatile to the Logback OpenTelemetry appender (#9748)

Author: jeanbisutti

instrumentation/logback/logback-appender-1.0/library/src/main/java/io/opentelemetry/instrumentation/logback/appender/v1_0/OpenTelemetryAppender.java

@@ -27,7 +27,7 @@ public class OpenTelemetryAppender extends UnsynchronizedAppenderBase<ILoggingEv
   private boolean captureKeyValuePairAttributes = false;
   private List<String> captureMdcAttributes = emptyList();
 
-  private OpenTelemetry openTelemetry;
+  private volatile OpenTelemetry openTelemetry;
   private LoggingEventMapper mapper;
 
   public OpenTelemetryAppender() {}