Review fixes for activej-http-6.0:javaagent (#16473)

Co-authored-by: otelbot <197425009+otelbot@users.noreply.github.com>

Author: otelbot[bot]

instrumentation/activej-http-6.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/activejhttp/ActivejAsyncServletInstrumentation.java

@@ -74,9 +74,9 @@ public static AdviceScope start(HttpRequest request) {
         return new AdviceScope(context, context.makeCurrent(), request);
       }
 
-      public Promise<HttpResponse> end(Promise<HttpResponse> responsePromise, Throwable throwable) {
+      public Promise<HttpResponse> end(
+          Promise<HttpResponse> responsePromise, @Nullable Throwable throwable) {
         scope.close();
-        Promise<HttpResponse> returnValue = responsePromise;
         if (throwable != null) {
           instrumenter().end(context, httpRequest, null, throwable);
           return responsePromise;

instrumentation/activej-http-6.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/activejhttp/ActivejHttpServerRequestGetter.java

@@ -16,17 +16,18 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import javax.annotation.Nullable;
 
-enum ActivejHttpServerRequestGetter implements TextMapGetter<HttpRequest> {
-  INSTANCE;
+final class ActivejHttpServerRequestGetter implements TextMapGetter<HttpRequest> {
 
   @Override
   public Iterable<String> keys(HttpRequest httpRequest) {
     return httpRequest.getHeaders().stream().map(h -> h.getKey().toString()).toList();
   }
 
+  @Nullable
   @Override
-  public String get(HttpRequest carrier, String key) {
+  public String get(@Nullable HttpRequest carrier, String key) {
     if (carrier == null) {
       return null;
     }
@@ -35,7 +36,7 @@ public String get(HttpRequest carrier, String key) {
   }
 
   @Override
-  public Iterator<String> getAll(HttpRequest carrier, String key) {
+  public Iterator<String> getAll(@Nullable HttpRequest carrier, String key) {
     if (carrier == null) {
       return emptyIterator();
     }

instrumentation/activej-http-6.0/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/activejhttp/ActivejHttpServerSingletons.java

@@ -21,7 +21,7 @@ public final class ActivejHttpServerSingletons {
         JavaagentHttpServerInstrumenters.create(
             INSTRUMENTATION_NAME,
             new ActivejHttpServerAttributesGetter(),
-            ActivejHttpServerRequestGetter.INSTANCE);
+            new ActivejHttpServerRequestGetter());
   }
 
   public static Instrumenter<HttpRequest, HttpResponse> instrumenter() {