from what i've found this is just netty noise... dosn't mean its not working. As my test with output to file showed.. the route in through input and out through output is working.
full netty log in case your interested
[2021-11-01T16:10:52,601][INFO ][logstash.javapipeline ] Pipeline Java execution initialization time {"seconds"=>1.27}
[2021-11-01T16:10:52,644][INFO ][logstash.inputs.beats ] Starting input listener {:address=>"0.0.0.0:2598"}
[2021-11-01T16:10:52,677][INFO ][logstash.javapipeline ] Pipeline started {"pipeline.id"=>"metricbeat"}
[2021-11-01T16:10:52,693][DEBUG][org.logstash.execution.PeriodicFlush] Pushing flush onto pipeline.
[2021-11-01T16:10:52,837][DEBUG][io.netty.util.internal.logging.InternalLoggerFactory] Using SLF4J as the default logging framework
[2021-11-01T16:10:52,845][DEBUG][io.netty.channel.MultithreadEventLoopGroup] -Dio.netty.eventLoopThreads: 8
[2021-11-01T16:10:52,894][DEBUG][io.netty.util.internal.InternalThreadLocalMap] -Dio.netty.threadLocalMap.stringBuilder.initialSize: 1024
[2021-11-01T16:10:52,896][DEBUG][io.netty.util.internal.InternalThreadLocalMap] -Dio.netty.threadLocalMap.stringBuilder.maxSize: 4096
[2021-11-01T16:10:52,917][DEBUG][io.netty.channel.nio.NioEventLoop] -Dio.netty.noKeySetOptimization: false
[2021-11-01T16:10:52,918][DEBUG][io.netty.channel.nio.NioEventLoop] -Dio.netty.selectorAutoRebuildThreshold: 512
[2021-11-01T16:10:52,996][DEBUG][io.netty.util.internal.PlatformDependent0] -Dio.netty.noUnsafe: false
[2021-11-01T16:10:52,996][DEBUG][io.netty.util.internal.PlatformDependent0] Java version: 11
[2021-11-01T16:10:52,998][DEBUG][io.netty.util.internal.PlatformDependent0] sun.misc.Unsafe.theUnsafe: available
[2021-11-01T16:10:52,999][DEBUG][io.netty.util.internal.PlatformDependent0] sun.misc.Unsafe.copyMemory: available
[2021-11-01T16:10:53,000][DEBUG][io.netty.util.internal.PlatformDependent0] java.nio.Buffer.address: available
[2021-11-01T16:10:53,000][DEBUG][io.netty.util.internal.PlatformDependent0] direct buffer constructor: unavailable: Reflective setAccessible(true) disabled
[2021-11-01T16:10:53,001][DEBUG][io.netty.util.internal.PlatformDependent0] java.nio.Bits.unaligned: available, true
[2021-11-01T16:10:53,005][DEBUG][io.netty.util.internal.PlatformDependent0] jdk.internal.misc.Unsafe.allocateUninitializedArray(int): unavailable: class io.netty.util.internal.PlatformDependent0$6 cannot access class jdk.internal.misc.Unsafe (in module java.base) because module java.base does not export jdk.internal.misc to unnamed module @3695ea5f
[2021-11-01T16:10:53,008][DEBUG][io.netty.util.internal.PlatformDependent0] java.nio.DirectByteBuffer.<init>(long, int): unavailable
[2021-11-01T16:10:53,014][DEBUG][io.netty.util.internal.PlatformDependent] sun.misc.Unsafe: available
[2021-11-01T16:10:53,018][DEBUG][io.netty.util.internal.PlatformDependent] maxDirectMemory: 1038876672 bytes (maybe)
[2021-11-01T16:10:53,021][DEBUG][io.netty.util.internal.PlatformDependent] -Dio.netty.tmpdir: /var/log/logstash/tmp (java.io.tmpdir)
[2021-11-01T16:10:53,022][DEBUG][io.netty.util.internal.PlatformDependent] -Dio.netty.bitMode: 64 (sun.arch.data.model)
[2021-11-01T16:10:53,026][DEBUG][io.netty.util.internal.PlatformDependent] -Dio.netty.maxDirectMemory: -1 bytes
[2021-11-01T16:10:53,027][DEBUG][io.netty.util.internal.PlatformDependent] -Dio.netty.uninitializedArrayAllocationThreshold: -1
[2021-11-01T16:10:53,034][DEBUG][io.netty.util.internal.CleanerJava9] java.nio.ByteBuffer.cleaner(): available
[2021-11-01T16:10:53,035][DEBUG][io.netty.util.internal.PlatformDependent] -Dio.netty.noPreferDirect: false
[2021-11-01T16:10:53,051][DEBUG][io.netty.util.internal.PlatformDependent] org.jctools-core.MpscChunkedArrayQueue: available
[2021-11-01T16:10:53,060][INFO ][org.logstash.beats.Server] Starting server on port: 2598
[2021-11-01T16:10:53,115][DEBUG][io.netty.channel.DefaultChannelId] -Dio.netty.processId: 7066 (auto-detected)
[2021-11-01T16:10:53,119][DEBUG][io.netty.util.NetUtil ] -Djava.net.preferIPv4Stack: false
[2021-11-01T16:10:53,119][DEBUG][io.netty.util.NetUtil ] -Djava.net.preferIPv6Addresses: false
[2021-11-01T16:10:53,123][DEBUG][io.netty.util.NetUtilInitializations] Loopback interface: lo (lo, 0:0:0:0:0:0:0:1%lo)
[2021-11-01T16:10:53,125][DEBUG][io.netty.util.NetUtil ] /proc/sys/net/core/somaxconn: 4096
[2021-11-01T16:10:53,126][DEBUG][io.netty.channel.DefaultChannelId] -Dio.netty.machineId: 00:50:56:ff:fe:b7:71:71 (auto-detected)
[2021-11-01T16:10:53,149][DEBUG][io.netty.util.ResourceLeakDetector] -Dio.netty.leakDetection.level: simple
[2021-11-01T16:10:53,149][DEBUG][io.netty.util.ResourceLeakDetector] -Dio.netty.leakDetection.targetRecords: 4
[2021-11-01T16:10:53,199][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.numHeapArenas: 8
[2021-11-01T16:10:53,199][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.numDirectArenas: 8
[2021-11-01T16:10:53,199][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.pageSize: 8192
[2021-11-01T16:10:53,200][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.maxOrder: 11
[2021-11-01T16:10:53,200][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.chunkSize: 16777216
[2021-11-01T16:10:53,200][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.smallCacheSize: 256
[2021-11-01T16:10:53,200][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.normalCacheSize: 64
[2021-11-01T16:10:53,200][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.maxCachedBufferCapacity: 32768
[2021-11-01T16:10:53,200][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.cacheTrimInterval: 8192
[2021-11-01T16:10:53,200][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.cacheTrimIntervalMillis: 0
[2021-11-01T16:10:53,201][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.useCacheForAllThreads: true
[2021-11-01T16:10:53,201][DEBUG][io.netty.buffer.PooledByteBufAllocator] -Dio.netty.allocator.maxCachedByteBuffersPerChunk: 1023
[2021-11-01T16:10:53,223][DEBUG][io.netty.buffer.ByteBufUtil] -Dio.netty.allocator.type: pooled
[2021-11-01T16:10:53,224][DEBUG][io.netty.buffer.ByteBufUtil] -Dio.netty.threadLocalDirectBufferSize: 0
[2021-11-01T16:10:53,224][DEBUG][io.netty.buffer.ByteBufUtil] -Dio.netty.maxThreadLocalCharBufferSize: 16384