reactor.netty.bytebuf.allocator.* metrics missing starting with Netty 4.2

[lukas-riedler-dynatrace]

Since Netty 4.2 there is a new default allocator: AdaptiveByteBufAllocator
See release notes: https://netty.io/news/2025/04/03/4-2-0.html

That allocator does not produce metrics.

With the update to Spring Boot 4 we lost these metrics for our applications, since that includes the update to Netty 4.2.

We use reactor-netty-core and http in version 1.3.5
And netty in version 4.2.12

Expected Behavior

We should still get these metrics. As far as I can tell this new allocator still allocates direct memory, and we'd like to see how much memory is being used by reactor-netty.
Unless I'm misunderstanding how the new allocator works, and it's not necessary any longer.

Actual Behavior

Metrics are lost.

Steps to Reproduce

Enable reactor netty metrics on a Spring Boot Webflux application, and check the reported metrics on actuator endpoint. Or debug the reactor.netty.transport.TransportConfig.TransportChannelInitializer#initChannel to see that the instanceof checks fail.

Use -Dio.netty.allocator.type=pooled then the pooled allocator, which is the previous default, is picked, and the metrics are there again.

Possible Solution

Add support in reactor.netty.transport.TransportConfig.TransportChannelInitializer#initChannel for io.netty.buffer.AdaptiveByteBufAllocator

Your Environment

• Reactor version(s) used: 1.3.5
• Other relevant libraries versions (eg. netty, ...): netty in version 4.2.12
• JVM version (java -version): Java 25
• OS and version (eg. uname -a): Locally for Ubuntu and in distroless container