JFR events support

[franz1981]

Fixes #5

I'm still working on:

• finalize the JFR events in a form which work OOTB with JMC (which is what I'm struggling with ATM)
• make sure this is not introducing any performance issue e.g. ATM I'm allocating a bit more on VT creation due to the SchedulingContext changes
• Eventually provide some jbang script which process the JFR events to produce additional telemetry/charts and actionable data
• Once JFR events are finalized write unit tests to make sure they work as intended

This is meant to understand if it makes sense to modify Netty to improve the "fairness" (see "Pareto Efficiency") of the scheduler vs I/O handling - which is supposed to be fast, but...better make sure it is.

In some later iteration I could provide an additional "context API" to allow binding the JFR events to HTTP request-related context - and even better - creating a custom scheduler for the purpose of instrumenting the built-in one as well too, and compare the two behaviours.

[Copilot]

Pull request overview

This PR adds comprehensive Java Flight Recorder (JFR) instrumentation to the Netty virtual thread scheduler, along with introducing FIFO and LIFO scheduler variants for performance comparison. The work includes custom JFR events for tracking scheduler activity, integration with the benchmark runner for capturing and analyzing JFR data, and a browser-based timeline viewer for visualizing scheduler behavior.

Changes:

• Introduces five custom JFR events (NettyRunIo, NettyRunTasks, VirtualThreadTaskRuns, VirtualThreadTaskRun, VirtualThreadTaskSubmit) to instrument scheduler activity
• Refactors EventLoopScheduler from a concrete class to an interface with FIFO and LIFO implementations
• Extends SchedulingContext to track whether tasks are pollers and include scheduler references
• Updates all tests to run against both FIFO and LIFO scheduler types
• Adds complete JFR integration to the benchmark runner including event capture, timeline export via JfrToTimeline.java, and an HTML-based timeline viewer

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
core/src/main/java/io/netty/loom/jfr/*.java	Five new JFR event definitions for scheduler instrumentation
core/src/main/java/io/netty/loom/SchedulerJfrUtil.java	Helper utility for begin/commit patterns on JFR events
core/src/main/java/io/netty/loom/EventLoopScheduler.java	Refactored from class to interface, with nested SharedRef and SchedulingContext classes
core/src/main/java/io/netty/loom/FifoEventLoopScheduler.java	New FIFO implementation with JFR instrumentation
core/src/main/java/io/netty/loom/LifoEventLoopScheduler.java	New LIFO implementation with JFR instrumentation
core/src/main/java/io/netty/loom/EventLoopSchedulerType.java	New enum for FIFO/LIFO scheduler types
core/src/main/java/io/netty/loom/VirtualMultithreadIoEventLoopGroup.java	Updated to support configurable scheduler type
core/src/main/java/io/netty/loom/NettyScheduler.java	Updated for new SchedulingContext structure
core/src/test/java/io/netty/loom/VirtualMultithreadIoEventLoopGroupTest.java	All tests parameterized to run against both scheduler types
benchmark-runner/src/main/java/io/netty/loom/benchmark/runner/HandoffHttpServer.java	Added scheduler type CLI parameter
benchmark-runner/scripts/run-benchmark.sh	Comprehensive JFR integration with event selection and timeline export
benchmark-runner/scripts/jfr/JfrToTimeline.java	jbang script to convert JFR recordings to compact JSONL timeline format
benchmark-runner/scripts/jfr/timeline-viewer.html	Browser-based interactive timeline visualization tool
benchmark-runner/scripts/jfr/netty-loom.jfc	Default JFR settings file for Netty Loom events
benchmark-runner/README.md	Documentation for JFR configuration and usage
README.md	Overview of available JFR events

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