Fix muzzle failure with webflux 7

instrumentation/spring/spring-webflux/spring-webflux-5.3/library/src/main/java/io/opentelemetry/instrumentation/spring/webflux/v5_3/internal/WebClientHttpAttributesGetter.java

@@ -8,8 +8,12 @@
 import static java.util.Collections.emptyList;
 
 import io.opentelemetry.instrumentation.api.semconv.http.HttpClientAttributesGetter;
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
 import java.util.List;
 import javax.annotation.Nullable;
+import org.springframework.http.HttpHeaders;
 import org.springframework.web.reactive.function.client.ClientRequest;
 import org.springframework.web.reactive.function.client.ClientResponse;
 
@@ -21,6 +25,28 @@ public enum WebClientHttpAttributesGetter
     implements HttpClientAttributesGetter<ClientRequest, ClientResponse> {
   INSTANCE;
 
+  private static final MethodHandle GET_HEADERS;
+
+  static {
+    // since webflux 7.0
+    MethodHandle methodHandle =
+        findGetHeadersMethod(MethodType.methodType(List.class, String.class, List.class));
+    if (methodHandle == null) {
+      // up to webflux 7.0
+      methodHandle =
+          findGetHeadersMethod(MethodType.methodType(Object.class, Object.class, Object.class));
+    }
+    GET_HEADERS = methodHandle;
+  }
+
+  private static MethodHandle findGetHeadersMethod(MethodType methodType) {
+    try {
+      return MethodHandles.lookup().findVirtual(HttpHeaders.class, "getOrDefault", methodType);
+    } catch (Throwable t) {
+      return null;
+    }
+  }
+
   @Override
   public String getUrlFull(ClientRequest request) {
     return request.url().toString();
@@ -32,8 +58,16 @@ public String getHttpRequestMethod(ClientRequest request) {
   }
 
   @Override
+  @SuppressWarnings("unchecked") // casting MethodHandle.invoke result
   public List<String> getHttpRequestHeader(ClientRequest request, String name) {
-    return request.headers().getOrDefault(name, emptyList());
+    if (GET_HEADERS != null) {
+      try {
+        return (List<String>) GET_HEADERS.invoke(request.headers(), name, emptyList());
+      } catch (Throwable t) {
+        // ignore
+      }
+    }
+    return emptyList();
   }
 
   @Override