Implements PR #3 from STEP_TRACING_PR_PLAN.md

Summary

Step-level observability with probabilistic sampling for vLLM scheduler.

Features

• CLI flags: --step-tracing-enabled, --step-tracing-sample-rate
• 16 span attributes: queue depths, batch composition, token counts, KV metrics
• Long-lived scheduler_steps span with step.BATCH_SUMMARY events
• Deterministic sampling for tests (hash-based)

Testing

• 9 comprehensive unit tests
• All 111/111 tests passing
• Zero regressions

Performance

• O(n) complexity (optimized from initial O(n²))
• Zero overhead when disabled
• Failure-safe (try/except wrapper)

Review Fixes Applied

• Fixed O(n²) → O(n) complexity with dict-based lookup
• Moved emission before _update_after_schedule() for spec compliance
• Removed dead code, explicit endpoint handling
• Gated timestamp capture, documented test assumptions

Files Changed

• 7 files changed, 1,462 insertions(+)
• 2 new files: STEP_TRACING_PR_PLAN.md, test_step_tracing.py
• 5 modified: config, CLI, constants, scheduler, test utils

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