fix compile error

Author: laurit

instrumentation/jetty/jetty-12.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v12_0/Jetty12Helper.java

@@ -8,7 +8,7 @@
 import io.opentelemetry.context.Context;
 import io.opentelemetry.instrumentation.api.instrumenter.Instrumenter;
 import io.opentelemetry.javaagent.bootstrap.servlet.AppServerBridge;
-import io.opentelemetry.javaagent.instrumentation.servlet.ServletHelper;
+import io.opentelemetry.javaagent.instrumentation.servlet.BaseServletHelper.ServletAsyncContext;
 import javax.annotation.Nullable;
 import org.eclipse.jetty.server.HttpStream;
 import org.eclipse.jetty.server.Request;
@@ -54,7 +54,7 @@ public void end(Context context, Request request, Response response, @Nullable T
       error = AppServerBridge.getException(context);
     }
     if (error == null) {
-      error = (Throwable) request.getAttribute(ServletHelper.ASYNC_EXCEPTION_ATTRIBUTE);
+      error = ServletAsyncContext.getAsyncException(context);
     }
 
     instrumenter.end(context, request, response, error);

instrumentation/servlet/servlet-common/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/servlet/BaseServletHelper.java

@@ -181,7 +181,7 @@ private static boolean sameTrace(Span oneSpan, Span otherSpan) {
     return oneSpan.getSpanContext().getTraceId().equals(otherSpan.getSpanContext().getTraceId());
   }
 
-  protected static class ServletAsyncContext implements ImplicitContextKeyed {
+  public static class ServletAsyncContext implements ImplicitContextKeyed {
     private static final ContextKey<ServletAsyncContext> CONTEXT_KEY =
         named("opentelemetry-servlet-async-context");