Fix server.request.body.filenames for in-memory uploads in Undertow 2.2

In undertow 2.2+, FormValueImpl.isFile() returns false for in-memory file uploads
(file size below fileSizeThreshold) because it checks fileItem.isInMemory(). Use
getFileName() to identify file uploads regardless of storage, which works across
all undertow versions. Also check the filenames callback before building the list
to avoid allocations on requests where the feature is inactive.

Author: jandro996

dd-java-agent/instrumentation/undertow/undertow-2.0/src/main/java/datadog/trace/instrumentation/undertow/FormDataMap.java

@@ -21,7 +21,9 @@ private Map<String, Collection<String>> getMap() {
       Deque<FormData.FormValue> formValues = formData.get(key);
       List<String> values = new ArrayList<>(formValues.size());
       for (FormData.FormValue formValue : formValues) {
-        if (!formValue.isFile()) {
+        // In undertow 2.2+, isFile() returns false for in-memory file uploads; getFileName()
+        // correctly identifies all file uploads regardless of storage.
+        if (formValue.getFileName() == null) {
           values.add(formValue.getValue());
         }
       }

dd-java-agent/instrumentation/undertow/undertow-2.0/src/main/java/datadog/trace/instrumentation/undertow/MultiPartUploadHandlerInstrumentation.java

@@ -106,21 +106,19 @@ static void after(
         }
       }
 
-      List<String> filenames = new ArrayList<>();
-      for (String key : attachment) {
-        for (FormData.FormValue formValue : attachment.get(key)) {
-          if (formValue.isFile()) {
+      BiFunction<RequestContext, List<String>, Flow<Void>> filenamesCb =
+          cbp.getCallback(EVENTS.requestFilesFilenames());
+      if (filenamesCb != null) {
+        List<String> filenames = new ArrayList<>();
+        for (String key : attachment) {
+          for (FormData.FormValue formValue : attachment.get(key)) {
             String filename = formValue.getFileName();
             if (filename != null && !filename.isEmpty()) {
               filenames.add(filename);
             }
           }
         }
-      }
-      if (!filenames.isEmpty()) {
-        BiFunction<RequestContext, List<String>, Flow<Void>> filenamesCb =
-            cbp.getCallback(EVENTS.requestFilesFilenames());
-        if (filenamesCb != null) {
+        if (!filenames.isEmpty()) {
           Flow<Void> filenamesFlow = filenamesCb.apply(reqCtx, filenames);
           Flow.Action filenamesAction = filenamesFlow.getAction();
           if (t == null && filenamesAction instanceof Flow.Action.RequestBlockingAction) {