Revert HelidonServerResponseMutator singleton; instantiate inline

Matches the dominant convention for non-enum HttpServerResponseMutator implementations in this repo (Akka, Pekko, Grizzly, Jetty 8/11/12, Servlet 3/5, Liberty all use 'new XyzResponseMutator()' at the call site) and the default rule in general-rules.md ('Prefer Instance Creation Over Singletons'). Header customization is not a hot path that warrants the singleton exception.

Author: trask

instrumentation/helidon-4.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/helidon/v4_3/HelidonServerResponseMutator.java

@@ -10,10 +10,6 @@
 
 final class HelidonServerResponseMutator implements HttpServerResponseMutator<ServerResponse> {
 
-  static final HelidonServerResponseMutator INSTANCE = new HelidonServerResponseMutator();
-
-  private HelidonServerResponseMutator() {}
-
   @Override
   public void appendHeader(ServerResponse res, String name, String value) {
     res.header(name, value);

instrumentation/helidon-4.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/helidon/v4_3/ResponseCustomizingFilter.java

@@ -21,7 +21,7 @@ public void filter(FilterChain chain, RoutingRequest req, RoutingResponse res) {
 
     var context = Context.current();
     HttpServerResponseCustomizerHolder.getCustomizer()
-        .customize(context, res, HelidonServerResponseMutator.INSTANCE);
+        .customize(context, res, new HelidonServerResponseMutator());
     chain.proceed();
   }
 }