fix: copy operationSpan to local variable in attemptStarted

Author: jinseopkim0

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/tracing/OpenTelemetryTracingTracer.java

@@ -181,8 +181,9 @@ public void attemptStarted(Object request, int attemptNumber) {
     // Attempt spans are of the CLIENT kind
     spanBuilder.setSpanKind(SpanKind.CLIENT);
 
-    if (operationSpan != null) {
-      spanBuilder.setParent(io.opentelemetry.context.Context.current().with(operationSpan));
+    Span localOperationSpan = operationSpan;
+    if (localOperationSpan != null) {
+      spanBuilder.setParent(io.opentelemetry.context.Context.current().with(localOperationSpan));
     }
 
     // Pass the combined attributes to the new SpanBuilder method