Fix Jetty 12 HTTP server metrics collection without virtual threads

After upgrading to Dropwizard 5 (which uses Jetty 12), HTTP server metrics
were not being collected or exposed correctly when virtual threads were
disabled (enableVirtualThreads=false). The metrics appeared correctly only
when virtual threads were enabled.

Root Cause:
The Jetty12ServerInstrumentation was ending spans in two places:
1. In onMethodExit when the handle() method completed
2. In HttpStream callbacks (succeeded/failed) for request completion

When virtual threads were disabled, handle() often completed synchronously
BEFORE the HttpStream callbacks fired. This caused the span to end prematurely
in onMethodExit, and metrics were not properly captured. The HttpStream
callbacks would then try to end an already-ended span.

When virtual threads were enabled, the asynchronous nature ensured callbacks
fired before method exit, so metrics were captured correctly.

Solution:
1. Modified Jetty12ServerInstrumentation.HandlerAdvice.end() to NOT end the
   span in onMethodExit (except when there's an exception). The span is now
   ended exclusively by the HttpStream completion callbacks.

2. Added AtomicBoolean in Jetty12Helper.start() to ensure the span ends
   exactly once, preventing double-ending issues regardless of callback order.

This ensures HTTP metrics (request counts, latency, body size, per-endpoint
and per-status-code metrics) are collected correctly whether virtual threads
are enabled or disabled.

Fixes the issue where Dropwizard 5 applications with enableVirtualThreads=false
would not report HTTP server metrics.

Author: Arunodoy18

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

@@ -9,6 +9,7 @@
 import io.opentelemetry.instrumentation.api.instrumenter.Instrumenter;
 import io.opentelemetry.javaagent.bootstrap.servlet.AppServerBridge;
 import io.opentelemetry.javaagent.bootstrap.servlet.ServletAsyncContext;
+import java.util.concurrent.atomic.AtomicBoolean;
 import javax.annotation.Nullable;
 import org.eclipse.jetty.server.HttpStream;
 import org.eclipse.jetty.server.Request;
@@ -27,21 +28,33 @@ public boolean shouldStart(Context parentContext, Request request) {
 
   public Context start(Context parentContext, Request request, Response response) {
     Context context = instrumenter.start(parentContext, request);
-    request.addFailureListener(throwable -> end(context, request, response, throwable));
+    // Use AtomicBoolean to ensure the span ends exactly once
+    AtomicBoolean spanEnded = new AtomicBoolean(false);
+
+    request.addFailureListener(
+        throwable -> {
+          if (spanEnded.compareAndSet(false, true)) {
+            end(context, request, response, throwable);
+          }
+        });
     // detect request completion
     // https://github.com/jetty/jetty.project/blob/52d94174e2c7a6e794c6377dcf9cd3ed0b9e1806/jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/handler/EventsHandler.java#L75
     request.addHttpStreamWrapper(
         stream ->
             new HttpStream.Wrapper(stream) {
               @Override
               public void succeeded() {
-                end(context, request, response, null);
+                if (spanEnded.compareAndSet(false, true)) {
+                  end(context, request, response, null);
+                }
                 super.succeeded();
               }
 
               @Override
               public void failed(Throwable throwable) {
-                end(context, request, response, throwable);
+                if (spanEnded.compareAndSet(false, true)) {
+                  end(context, request, response, throwable);
+                }
                 super.failed(throwable);
               }
             });

instrumentation/jetty/jetty-12.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v12_0/Jetty12ServerInstrumentation.java

@@ -67,6 +67,10 @@ public static AdviceScope start(Object source, Request request, Response respons
 
       public void end(Request request, Response response, @Nullable Throwable throwable) {
         scope.close();
+        // Don't end the span here - it will be ended by the HttpStream callbacks
+        // registered in Jetty12Helper.start(). This ensures metrics are captured
+        // correctly regardless of whether virtual threads are enabled.
+        // Only end immediately if there's an exception, as the callbacks may not fire.
         if (throwable != null) {
           helper().end(context, request, response, throwable);
         }