feat(metrics): add task completion latency tracking and reporting

[staryxchen]

Add Transfer Task Latency Distribution Metrics

Summary

This PR adds histogram-based latency distribution tracking for transfer tasks in the Transfer Engine metrics system.

Motivation

Understanding task completion latency distribution is crucial for:

• Performance tuning and bottleneck identification
• SLA monitoring and analysis
• Detecting outliers and tail latencies

Previously, only throughput metrics were available. This enhancement provides detailed visibility into task-level performance.

Changes

Core Implementation

Modified Files:

• mooncake-transfer-engine/include/transfer_engine.h
• mooncake-transfer-engine/src/transfer_engine.cpp

Key Components:

1. Latency Tracking

   • Record task start time on first getTransferStatus() call
   • Calculate and record completion latency when task completes
   • Use ylt::metric::histogram_t for efficient distribution tracking

2. Histogram Buckets

   • Fine-grained buckets (10μs - 10s) covering sub-millisecond to multi-second latencies
   • 17 boundary values creating 18 buckets optimized for high-performance scenarios

3. Interval-based Reporting

   • Snapshot mechanism for computing per-interval statistics
   • Avoids cumulative drift, shows actual distribution within each reporting period
   • Aligns with Prometheus-style metrics design (monotonic counters + rate calculation)

Output Format

[Metrics] Transfer Engine Stats (over last 5s): 
Throughput: 22480.90 MB/s | 
Latency Distribution (count=56215): 0-10μs:0.2%, 10-20μs:1.6%, 20-50μs:96.5%, 50-100μs:0.9%, 100-200μs:0.8%

Features:

• Only shows buckets with ≥0.1% to reduce noise
• Displays task count and percentage distribution
• Single-line format for easy parsing and monitoring

Usage

# Enable metrics (required)
export MC_TE_METRIC=1

# Optional: customize interval (default: 5s)
export MC_TE_METRIC_INTERVAL_SECONDS=10

Design Decisions

1. Histogram over avg/max: Distribution provides complete picture; avg can be skewed by tail latencies, max is unstable
2. Snapshot mechanism: histogram_t doesn't support reset; snapshot-based interval calculation is standard practice (Prometheus)
3. Bucket granularity: Focused on sub-ms to catch high-performance variations, with coarser bins for tail

Performance Impact

• Minimal overhead: only enabled with WITH_METRICS compile flag
• Lock-free histogram updates
• Automatic cleanup of completed task timestamps

---

Testing Instructions:

# Build with metrics
cmake -DWITH_METRICS=ON ..
make

# Run and observe output
export MC_TE_METRIC=1
./your_test_binary

Expected output will show latency distribution every 5 seconds alongside throughput metrics.

Type of Change

• Types
  • [ ] Bug fix
  • [x] New feature
    • [x] Transfer Engine
    • [ ] Mooncake Store
    • [ ] Mooncake EP
    • [ ] Integration
    • [ ] P2P Store
    • [ ] Python Wheel
  • [ ] Breaking change
  • [ ] CI/CD
  • [ ] Documentation update
  • [ ] Other

How Has This Been Tested?

• ✅ Compiles with WITH_METRICS enabled/disabled
• ✅ No memory leaks (completed tasks are cleaned up)
• ✅ Thread-safe concurrent task tracking
• ✅ Accurate distribution reporting under load

Checklist

• [x] I have performed a self-review of my own code.
• [x] I have updated the documentation.
• [x] I have added tests to prove my changes are effective.

[gemini-code-assist[bot]]

Summary of Changes

Hello @staryxchen, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces comprehensive task completion latency distribution metrics to the Transfer Engine. This enhancement moves beyond simple throughput measurements to provide critical insights into task-level performance, enabling better performance tuning, more accurate SLA monitoring, and improved identification of tail latencies and bottlenecks.

Highlights

• Task Completion Latency Tracking: Implemented histogram-based latency distribution tracking for transfer tasks within the Transfer Engine metrics system.
• Detailed Latency Metrics: Records task start times on the first getTransferStatus() call and calculates completion latency when a task finishes, utilizing ylt::metric::histogram_t for efficient distribution tracking.
• Configurable Histogram Buckets: Uses 17 fine-grained boundary values (10μs to 10s) to cover sub-millisecond to multi-second latencies, optimized for high-performance scenarios.
• Interval-based Reporting: Employs a snapshot mechanism to compute and report per-interval latency statistics, avoiding cumulative drift and aligning with Prometheus-style metrics design.
• Enhanced Metrics Output: The metrics output now includes both throughput and a detailed latency distribution, displaying only buckets with >=0.1% to reduce noise and provide a clear, single-line format.

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[stmatengss]

👍 I will review it today.