feature: benchmark CLI - add P99.9 latency percentile

[WangzJi]

Check Ahead

• I have searched the issues of this repository and believe that this is not a duplicate.
• I am willing to try to implement this feature myself.

Why you need it?

Current LatencyStats only reports P50/P95/P99/Max. P99.9 is critical for tail latency analysis in distributed transaction systems and is a standard metric in performance benchmarking tools (e.g., Kafka ProducerPerformance).

How it could be?

• Add p999 field to BenchmarkMetrics.LatencyStats
• Update getLatencyStats() to calculate P99.9 percentile
• Update MetricsCollector.generateFinalReport() to include P99.9 in the output
• Update MetricsCollector.exportToCsv() to include P99.9 row

[DoChaoing]

I would like to work on this issue. I will submit a PR soon.