Netty - Add Gauges for Direct Memory Usage & Max

Is your feature request related to a problem? Please describe.

Once I moved my apps to Java 25, and Netty (4.2.16) stopped using Direct Buffers, I stopped seeing the off-heap usage via the jvm.buffer.memory.usage & jvm.buffer.memory.used metrics.

It seems like if available, Netty links against malloc and free for performance reasons.

Describe the solution you'd like

Updated or new metrics for tracking the usage and maximum for Netty's direct memory usage.

Describe alternatives you've considered

• Not having these metrics
• Performing manual instrumentation in every Java application I run

Additional context

I was able to get around this temporarily in 1 app with the below custom gauges.

private val usedDirectMemory = meter.gaugeBuilder("netty.native.used.direct.memory")
    .setDescription("Used direct memory in bytes")
    .setUnit("bytes")
    .buildWithCallback { result ->
        val usedMemory = io.netty.util.internal.PlatformDependent.usedDirectMemory()
        result.record(usedMemory.toDouble())
    }

private val maxDirectMemory = meter.gaugeBuilder("netty.native.max.direct.memory")
    .setDescription("Max direct memory in bytes")
    .setUnit("bytes")
    .buildWithCallback { result ->
        val maxMemory = io.netty.util.internal.PlatformDependent.maxDirectMemory()
        result.record(maxMemory.toDouble())
    }

Tip

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Start by reviewing the existing instrumentation around Netty's PlatformDependent.usedDirectMemory() and maxDirectMemory() entry points, using the Kotlin custom gauges as the requested behavior. Add metrics for current and maximum direct memory usage, with byte units and descriptions matching the example, and verify that both values are exposed.