Fix memory leak in http pipelining

This is related to elastic#30801. When we calling the http pipelining
aggregator on an inbound request, we retain the netty request. However,
this is unnecessary as the pipelining aggregator does not store the
request. This worked in the past as we manually release the request and
netty internally automatically releases the request. At this point we do
not implement the ref counter interface after the pipelining step which
means that netty is no longer automatically handling this second retain.

This commit removes that retain.

modules/transport-netty4/src/main/java/org/elasticsearch/http/netty4/Netty4HttpPipeliningHandler.java

@@ -56,7 +56,7 @@ public Netty4HttpPipeliningHandler(Logger logger, final int maxEventsHeld) {
     @Override
     public void channelRead(final ChannelHandlerContext ctx, final Object msg) {
         if (msg instanceof LastHttpContent) {
-            HttpPipelinedRequest<LastHttpContent> pipelinedRequest = aggregator.read(((LastHttpContent) msg).retain());
+            HttpPipelinedRequest<LastHttpContent> pipelinedRequest = aggregator.read(((LastHttpContent) msg));
             ctx.fireChannelRead(pipelinedRequest);
         } else {
             ctx.fireChannelRead(msg);

plugins/transport-nio/src/main/java/org/elasticsearch/http/nio/NioHttpPipeliningHandler.java

@@ -56,7 +56,7 @@ public NioHttpPipeliningHandler(Logger logger, final int maxEventsHeld) {
     @Override
     public void channelRead(final ChannelHandlerContext ctx, final Object msg) {
         if (msg instanceof LastHttpContent) {
-            HttpPipelinedRequest<LastHttpContent> pipelinedRequest = aggregator.read(((LastHttpContent) msg).retain());
+            HttpPipelinedRequest<LastHttpContent> pipelinedRequest = aggregator.read(((LastHttpContent) msg));
             ctx.fireChannelRead(pipelinedRequest);
         } else {
             ctx.fireChannelRead(msg);