Intermittent metric registration error: worker_millis_per_event

Problem:

Intermittent warning during pipeline startup. The worker_millis_per_event metric fails to register but pipeline functions normally.

[org.logstash.execution.AbstractPipelineExt] Metric registration error: `worker_millis_per_event` could not be registered in namespace `[:stats, :pipelines, :logstash, :plugins, :filters, :c13c16377fd9a15e6e691284665f2da3494b42273b1fc6e37b6330fff12eb5ab, :flow]

Expected versus actual behavior:

Expected: All flow metrics register successfully; /_node/stats/pipelines returns full pipeline statistics.

Actual: Registration fails. Pipelines start and process events normally (status: "green", Pipelines running {count: 2}). However, /_node/stats/pipelines returns empty pipeline data: {"pipelines": {}}.

Environment Details:

1. Logstash version - 9.0.3 OSS Version
2. Logstash installation source (e.g. built from source, with a package manager: DEB/RPM, expanded from tar or zip archive, docker) - Built from source
3. How is Logstash being run (e.g. as a service/service manager: systemd, upstart, etc. Via command line, docker/kubernetes) - Kubernetes (running as a process with logstash binary in Docker container)

JVM:

bash-4.4$ java -version openjdk version "21.0.12" 2026-07-21 OpenJDK Runtime Environment (build 21.0.12+8-suse-01500-x8664) OpenJDK 64-Bit Server VM (build 21.0.12+8-suse-01500-x8664, mixed mode, sharing)

OS version:

Linux log-transformer-597b585774 6.4.0-150600.23.42-default #1 SMP PREEMPT_DYNAMIC Fri Mar 7 09:53:00 UTC 2025 (7bf6ecd) x86_64 x86_64 x86_64 GNU/Linux

Steps to reproduce:

Not consistently reproducible - Occurs intermittently during startup. The reported error is observed approximately once in every 40-50 startups. Reproduced with automated script that deploys logstash, searches for error pattern during startup, redeploys if error not found. This continues till error pattern is found.

Additional Observations:

1. No functional impact - Pipeline processes events normally

2. Metrics not available - Pipeline stats are empty from endpoint localhost:9600/_node/stats/pipelines { "host": "log-transformer", "version": "9.0.3", "http_address": "0.0.0.0:9600", "id": "5fbd8fxxxxxxxxxxxx5602", "name": "log-transformer", "ephemeral_id": "36axxxxxxxxxxxxxxxxx2348a7", "snapshot": false, "status": "unknown", "pipeline": { "workers": 2, "batch_size": 2048, "batch_delay": 50 }, "pipelines": {} }

3. Self-resolving: Logstash process restart with same configuration succeeds without error. No warning message and stats are also available in /_node/stats/pipelines endpoint.

Note - We have created a discussion in elastic community (https://discuss.elastic.co/t/intermittent-metric-registration-error-worker-millis-per-event/383836) and tried the workaround/suggestions provided there. But unfortunately, it didn't solve our issue.

Logs:

Full startup logs attached - Logstash Logs - Metrics registration error · GitHub