Review fixes for netty-3.8:javaagent (#18222)

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

Author: otelbot[bot]

instrumentation/netty/netty-3.8/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v3_8/client/HttpRequestHeadersSetter.java

@@ -9,8 +9,7 @@
 import io.opentelemetry.javaagent.instrumentation.netty.v3_8.NettyRequest;
 import javax.annotation.Nullable;
 
-enum HttpRequestHeadersSetter implements TextMapSetter<NettyRequest> {
-  INSTANCE;
+final class HttpRequestHeadersSetter implements TextMapSetter<NettyRequest> {
 
   @Override
   public void set(@Nullable NettyRequest carrier, String key, String value) {

instrumentation/netty/netty-3.8/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v3_8/client/NettyClientSingletons.java

@@ -30,7 +30,7 @@ public class NettyClientSingletons {
         JavaagentHttpClientInstrumenters.create(
             INSTRUMENTATION_NAME,
             new NettyHttpClientAttributesGetter(),
-            HttpRequestHeadersSetter.INSTANCE,
+            new HttpRequestHeadersSetter(),
             builder ->
                 builder.addContextCustomizer(
                     (context, requestAndChannel, startAttributes) ->
@@ -40,10 +40,10 @@ public class NettyClientSingletons {
         Instrumenter.<NettyConnectionRequest, Channel>builder(
                 GlobalOpenTelemetry.get(), INSTRUMENTATION_NAME, NettyConnectionRequest::spanName)
             .addAttributesExtractor(
-                HttpClientAttributesExtractor.create(NettyConnectHttpAttributesGetter.INSTANCE))
+                HttpClientAttributesExtractor.create(new NettyConnectHttpAttributesGetter()))
             .addAttributesExtractor(
                 HttpClientServicePeerAttributesExtractor.create(
-                    NettyConnectHttpAttributesGetter.INSTANCE, GlobalOpenTelemetry.get()))
+                    new NettyConnectHttpAttributesGetter(), GlobalOpenTelemetry.get()))
             .setSchemaUrl(SchemaUrls.V1_37_0)
             .buildInstrumenter(SpanKindExtractor.alwaysClient());
   }

instrumentation/netty/netty-3.8/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v3_8/client/NettyConnectHttpAttributesGetter.java

@@ -16,9 +16,8 @@
 import javax.annotation.Nullable;
 import org.jboss.netty.channel.Channel;
 
-enum NettyConnectHttpAttributesGetter
+final class NettyConnectHttpAttributesGetter
     implements HttpClientAttributesGetter<NettyConnectionRequest, Channel> {
-  INSTANCE;
 
   @Nullable
   @Override

instrumentation/netty/netty-3.8/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v3_8/server/NettyHeadersGetter.java

@@ -12,8 +12,7 @@
 import java.util.Iterator;
 import javax.annotation.Nullable;
 
-enum NettyHeadersGetter implements TextMapGetter<NettyRequest> {
-  INSTANCE;
+final class NettyHeadersGetter implements TextMapGetter<NettyRequest> {
 
   @Override
   public Iterable<String> keys(NettyRequest requestAndChannel) {
@@ -22,11 +21,11 @@ public Iterable<String> keys(NettyRequest requestAndChannel) {
 
   @Nullable
   @Override
-  public String get(@Nullable NettyRequest requestAndChannel, String s) {
+  public String get(@Nullable NettyRequest requestAndChannel, String key) {
     if (requestAndChannel == null) {
       return null;
     }
-    return requestAndChannel.request().headers().get(s);
+    return requestAndChannel.request().headers().get(key);
   }
 
   @Override

instrumentation/netty/netty-3.8/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v3_8/server/NettyServerSingletons.java

@@ -20,7 +20,7 @@ final class NettyServerSingletons {
         JavaagentHttpServerInstrumenters.create(
             "io.opentelemetry.netty-3.8",
             new NettyHttpServerAttributesGetter(),
-            NettyHeadersGetter.INSTANCE,
+            new NettyHeadersGetter(),
             builder ->
                 builder.addContextCustomizer(
                     (context, requestAndChannel, startAttributes) ->