create instrumenter directly, aligning code between agent and library

Author: zeitlinger

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

@@ -31,7 +31,7 @@ public final class NettyClientSingletons {
         JavaagentHttpClientInstrumenterBuilder.createWithCustomizer(
             INSTRUMENTATION_NAME,
             new NettyHttpClientAttributesGetter(),
-            Optional.empty(),
+            Optional.of(HttpRequestHeadersSetter.INSTANCE),
             builder -> {
               builder.addContextCustomizer(
                   (context, requestAndChannel, startAttributes) -> NettyErrorHolder.init(context));