Use static INSTANCE singleton for stateless HttpServerResponseMutators

instrumentation/grizzly-2.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/grizzly/v2_3/GrizzlyHttpResponseMutator.java

@@ -10,7 +10,8 @@
 import org.glassfish.grizzly.http.util.DataChunk;
 import org.glassfish.grizzly.http.util.MimeHeaders;
 
-public class GrizzlyHttpResponseMutator implements HttpServerResponseMutator<HttpResponsePacket> {
+public enum GrizzlyHttpResponseMutator implements HttpServerResponseMutator<HttpResponsePacket> {
+  INSTANCE;
 
   @Override
   public void appendHeader(HttpResponsePacket response, String name, String value) {

instrumentation/grizzly-2.3/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/grizzly/v2_3/HttpServerFilterInstrumentation.java

@@ -50,7 +50,7 @@ public static void onEnter(
         @Advice.Argument(2) HttpResponsePacket response) {
       Context context = GrizzlyStateStorage.getContext(ctx);
       HttpServerResponseCustomizerHolder.getCustomizer()
-          .customize(context, response, new GrizzlyHttpResponseMutator());
+          .customize(context, response, GrizzlyHttpResponseMutator.INSTANCE);
     }
 
     @Advice.OnMethodExit(onThrowable = Throwable.class, suppress = Throwable.class, inline = false)

instrumentation/jetty/jetty-12.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v12_0/Jetty12ResponseMutator.java

@@ -8,7 +8,8 @@
 import io.opentelemetry.javaagent.bootstrap.http.HttpServerResponseMutator;
 import org.eclipse.jetty.server.Response;
 
-class Jetty12ResponseMutator implements HttpServerResponseMutator<Response> {
+enum Jetty12ResponseMutator implements HttpServerResponseMutator<Response> {
+  INSTANCE;
 
   @Override
   public void appendHeader(Response response, String name, String value) {

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

@@ -61,7 +61,7 @@ public static AdviceScope start(Request request, Response response) {
         Context context = helper().start(parentContext, request, response);
         Scope scope = context.makeCurrent();
         HttpServerResponseCustomizerHolder.getCustomizer()
-            .customize(context, response, new Jetty12ResponseMutator());
+            .customize(context, response, Jetty12ResponseMutator.INSTANCE);
         return new AdviceScope(context, scope);
       }
 

instrumentation/jetty/jetty-8.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v8_0/Jetty8InstrumentationModule.java

@@ -78,7 +78,7 @@ public static AdviceScope start(HttpServletRequest request, HttpServletResponse
         // Must be set here since Jetty handlers can use startAsync outside of servlet scope.
         helper().setAsyncListenerResponse(context, response);
         HttpServerResponseCustomizerHolder.getCustomizer()
-            .customize(context, response, new Jetty8ResponseMutator());
+            .customize(context, response, Jetty8ResponseMutator.INSTANCE);
         return new AdviceScope(requestContext, context, scope);
       }
 

instrumentation/jetty/jetty-8.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/jetty/v8_0/Jetty8ResponseMutator.java

@@ -8,7 +8,9 @@
 import io.opentelemetry.javaagent.bootstrap.http.HttpServerResponseMutator;
 import javax.servlet.http.HttpServletResponse;
 
-final class Jetty8ResponseMutator implements HttpServerResponseMutator<HttpServletResponse> {
+enum Jetty8ResponseMutator implements HttpServerResponseMutator<HttpServletResponse> {
+  INSTANCE;
+
   @Override
   public void appendHeader(HttpServletResponse response, String name, String value) {
     response.addHeader(name, value);

instrumentation/liberty/liberty-20.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/liberty/v20_0/LibertyWebAppInstrumentation.java

@@ -129,7 +129,8 @@ public static void onEnter() {
       helper().setAsyncListenerResponse(context, requestInfo.getResponse());
 
       HttpServerResponseCustomizerHolder.getCustomizer()
-          .customize(context, requestInfo.getResponse(), new Servlet3HttpServerResponseMutator());
+          .customize(
+              context, requestInfo.getResponse(), Servlet3HttpServerResponseMutator.INSTANCE);
     }
   }
 }

instrumentation/servlet/servlet-3.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/servlet/v3_0/Servlet3HttpServerResponseMutator.java

@@ -8,10 +8,9 @@
 import io.opentelemetry.javaagent.bootstrap.http.HttpServerResponseMutator;
 import javax.servlet.http.HttpServletResponse;
 
-public class Servlet3HttpServerResponseMutator
+public enum Servlet3HttpServerResponseMutator
     implements HttpServerResponseMutator<HttpServletResponse> {
-  public static final Servlet3HttpServerResponseMutator INSTANCE =
-      new Servlet3HttpServerResponseMutator();
+  INSTANCE;
 
   @Override
   public void appendHeader(HttpServletResponse response, String name, String value) {