[FEAT] Add replay from trace strategy

[VincentG1234]

Summary

• Add a new replay benchmarking strategy that reproduces real-world request patterns from trace log files (.jsonl)
• Enable time-based request rate replay with precise timestamp scheduling
• Support synthetic prompt generation that matches token counts from trace files
• use max_requests and max_seconds cli options to limit the number of requests processed from a trace

Motivation

This change addresses issue #597 by enabling users to benchmark their vLLM servers using real production traces. Instead of synthetic load patterns, users can now replay exact request arrival times and token distributions from their actual workloads for more realistic performance testing.

Changes

• Add TraceReplayStrategy scheduler strategy for timestamp-based request dispatching
• Add ReplayProfile class for configuring trace-based benchmarking parameters
• Add TraceSyntheticDatasetDeserializer to generate prompts matching trace input/output lengths
• Add TraceReader utility for reading .jsonl trace files with timestamp, input_length, output_length fields
• Update Entrypoint to handle replay profile and dataset configuration
• use max_requests and max_seconds truncation support to limit trace replay length

Testing

• pytest tests/unit/scheduler/test_trace_replay.py (pass)

• pytest tests/unit/benchmark/test_replay_profile.py (pass)

• pytest tests/unit/data/deserializers/test_trace_synthetic.py (pass)

• Added tests: scheduling accuracy, boundary conditions, malformed trace handling, empty trace cases, max_requests truncation

• test it in practice quickly with NB COLAB

Notes / Risks

• Minimal implementation focused on core trace replay functionality
• Requires manual verification with real trace files before merging

Next Steps (this PR)

1. Apply reviewer feedback and address any comments
2. Add handwritten integration tests with sample trace files
3. Add E2E tests verifying end-to-end trace replay flow
4. Add CLI usage examples in PR description and docs
5. Update documentation with trace replay guide

Out of Scope (future PRs)

• Mooncake trace format support (token-level traces)
• Helper utilities for timestamp format conversions (Unix epoch, ISO8601, relative timestamps)
• Support for request payload traces (not just token counts)
• Trace file validation and schema verification tools
• Performance optimizations for large trace files (streaming, chunked processing)
• Metrics export formatted for trace analysis comparison
• Support for trace file compression formats (.gz, .bz2)