Make async-profiler per-thread profiling opt-in for Java

[artursarlo]

Make async-profiler per-thread profiling opt-in for Java

Summary

Per-thread profiling — async-profiler's threads output option, which splits collapsed stacks by thread name — was hard-coded on in PR #78 via FORMAT_PARAMS = "ann,sig,threads". That output is needed for Spark task attribution but adds cardinality/overhead that isn't desirable for every workload.

This PR makes per-thread profiling opt-in and disabled by default, controllable both from the CLI and from the heartbeat / command-control system. This is the agent-side change; the backend counterpart (surfacing the per_thread toggle in the profiling command) will be implemented against this PR.

Changes

• gprofiler/profilers/java.py
  • Reverted FORMAT_PARAMS back to "ann,sig".
  • Added per_thread: bool = False to AsyncProfiledProcess; when set it appends ,threads to the async-profiler output args (same pattern as includemm / includeln).
  • New CLI flag --java-async-profiler-per-thread (store_true, default False), threaded through JavaProfiler.
• gprofiler/dynamic_profiling_management/__init__.py
  • _apply_profiler_configs now reads an optional per_thread key from the async_profiler command config (default False) and sets java_async_profiler_per_thread.
• docs/HEARTBEAT_SYSTEM_README.md — documented the new per_thread field and added an example.
• tests/utils.py — threaded java_async_profiler_per_thread through make_java_profiler.

How to enable

• CLI: pass --java-async-profiler-per-thread.
• Heartbeat/command config:

  { "async_profiler": { "enabled": true, "time": "cpu", "per_thread": true } }

Behavior change

Default flamegraphs no longer include per-thread splitting. Deployments relying on per-thread output (e.g. Spark task attribution) must opt in via the flag or command config.

Backend follow-up

The backend needs to accept and forward the per_thread boolean in the async_profiler profiler config so it reaches the agent through the heartbeat response (default false for backward compatibility).

Test plan

• Unit: assert AP output args include/exclude ,threads based on the flag.
• Verify heartbeat command with per_thread: true/false produces expected flamegraphs.