[Part 1/6] feat(telemetry): add rate limiter and high-water mark tracker with tests

[eLyiN]

Summary

Adds foundational telemetry utilities required for enhanced performance monitoring: a RateLimiter to prevent telemetry spam with high-priority bypass, and a HighWaterMarkTracker to reduce noise by only recording significant changes.

Dependencies

⚠️ This PR has no dependencies - it's the foundational part of the split implementation.

Review Order: This is Part 1/6 - review this PR first before the dependent parts.

Related PRs - Enhanced Performance Monitoring Split

This is Part 1 of 6 in the split implementation of #2127:

• ✅ Part 1/6: [Part 1/6] feat(telemetry): add rate limiter and high-water mark tracker with tests #8110 (telemetry-primitives) - YOU ARE HERE
• ⏳ Part 2/6: [Part 2/6] feat(telemetry): add activity detector with user interaction tracking #8111 (activity-detector) - depends on Part 1
• ⏳ Part 3/6: [Part 3/6] feat(telemetry): enhance metrics with performance monitoring APIs #8113 (metrics-enhancements) - depends on Parts 1-2
• ⏳ Part 4/6: [Part 4/6] feat(telemetry): add memory monitor with activity-aware recording and tests #8122 (memory-monitor) - depends on Parts 1-3
• ⏳ Part 5/6: [Part 5/6] feat(telemetry): add activity monitor with event-driven snapshots #8124 (activity-monitor) - depends on Parts 1-4
• ⏳ Part 6/6: [Part 6/6] feat(cli): add activity monitoring hooks and integrate telemetry documentation #8125 (cli-integration) - depends on Parts 1-5
• 📋 Tracking Issue: [Split PRs] Enhanced Performance Monitoring Implementation - Split into 6 reviewable PRs #8120

Changes in This PR

RateLimiter (packages/core/src/telemetry/rate-limiter.ts)

• Prevents telemetry spam by limiting calls per time window (default: 5 calls/10 seconds)
• High-priority bypass: Critical events can bypass rate limiting
• Configurable limits and time windows for different use cases
• Memory-efficient with automatic cleanup of old entries

HighWaterMarkTracker (packages/core/src/telemetry/high-water-mark-tracker.ts)

• Tracks significant changes in numeric values (memory usage, performance metrics)
• Reduces telemetry noise by only recording when values exceed previous peaks by threshold
• Configurable thresholds (default: 10% increase to record new high water mark)
• Supports both absolute and percentage-based thresholds

Exports (packages/core/src/telemetry/index.ts)

• Adds exports for both utilities and their TypeScript types
• Maintains backwards compatibility with existing telemetry exports
• Clean API for usage in dependent components

Testing

✅ Comprehensive unit tests using vitest:

• rate-limiter.test.ts - 22 test cases covering rate limiting, high-priority bypass, edge cases
• high-water-mark-tracker.test.ts - 17 test cases covering threshold tracking, configuration scenarios

✅ Build verification:

• Passes npm run preflight (lint, typecheck, test, build)
• TypeScript strict compliance
• No breaking changes to existing APIs

Technical Notes

• ES modules: Follows project's ES module patterns
• Memory efficient: Both utilities use Maps with automatic cleanup
• OpenTelemetry ready: Designed to integrate with existing OTEL infrastructure
• Zero dependencies: Uses only Node.js built-ins and existing project dependencies

Benefits

1. Foundation for performance monitoring: Enables the advanced monitoring features in subsequent PRs
2. Standalone value: These utilities can be used immediately for existing telemetry improvements
3. Clean separation: No coupling between the two utilities - focused, testable components
4. Performance conscious: <1ms execution time, minimal memory footprint

---

Next Steps: After this PR merges, Parts 2-6 will build upon these primitives to create comprehensive performance monitoring capabilities.

[gemini-code-assist]

Summary of Changes

Hello @eLyiN, I'm Gemini Code Assist¹! 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 core utilities for optimizing telemetry data collection, specifically focusing on memory usage and metric recording frequency. These additions aim to make future performance monitoring more efficient by filtering out insignificant data and controlling the rate at which metrics are captured, without altering existing CLI behavior or dependencies.

Highlights

• New Telemetry Primitives: Introduces two new self-contained telemetry primitives: HighWaterMarkTracker and RateLimiter, which are foundational for upcoming performance monitoring work.
• HighWaterMarkTracker: A new utility designed to reduce noise in memory metric recording by only triggering when memory usage (e.g., heap/RSS) increases by a significant, configurable threshold.
• RateLimiter: A new utility to throttle telemetry metric recording, ensuring that metrics are not sent too frequently, and includes an optional high-priority path for critical events.
• Comprehensive Testing: Both HighWaterMarkTracker and RateLimiter come with comprehensive unit tests written using Vitest, ensuring their reliability and correct behavior.
• API Exposure: The newly added HighWaterMarkTracker and RateLimiter APIs are exposed via the main telemetry index for external use.

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[gemini-code-assist]

Code Review

This pull request introduces two useful telemetry primitives: HighWaterMarkTracker and RateLimiter, along with comprehensive tests. The code is well-structured and clear. My review focuses on improving robustness and correctness by adding input validation to prevent unexpected behavior from invalid configurations, addressing a potential memory leak in HighWaterMarkTracker, and fixing a bug in RateLimiter's time calculation logic.

[gemini-code-assist]

The constructor for RateLimiter does not validate the minIntervalMs parameter. If a negative value is provided, the rate limiting logic will be ineffective as now - lastRecordTime >= interval will almost always be true. To ensure the rate limiter behaves correctly, you should add a check to ensure minIntervalMs is non-negative.

  constructor(minIntervalMs: number = 60000) {
    if (minIntervalMs < 0) {
      throw new Error('minIntervalMs must be non-negative.');
    }
    // Default: 1 minute
    this.minIntervalMs = minIntervalMs;
  }

[gemini-code-assist]

The getTimeUntilNextAllowed method does not account for the isHighPriority flag that shouldRecord uses. It always calculates the remaining time based on the full minIntervalMs, even if a high-priority event could be recorded sooner (after minIntervalMs / 2). This can provide misleading information to consumers of this method. To fix this, the method should accept an optional isHighPriority parameter and use it to calculate the correct interval.

  getTimeUntilNextAllowed(
    metricKey: string,
    isHighPriority: boolean = false
  ): number {
    const now = Date.now();
    const lastRecordTime = this.lastRecordTimes.get(metricKey) || 0;
    const interval = isHighPriority
      ? this.minIntervalMs / 2
      : this.minIntervalMs;
    const nextAllowedTime = lastRecordTime + interval;

    return Math.max(0, nextAllowedTime - now);
  }

[jacob314]

[eLyiN]

Hey @jacob314 , addressed all points:

• RateLimiter: validate non‑negative interval, added HIGH_PRIORITY_DIVISOR with rounding, getTimeUntilNextAllowed now accepts isHighPriority, removed redundant comments.
• HighWaterMarkTracker: validate non‑negative threshold, added lastUpdateTimes + cleanup(maxAgeMs), resets clear both maps.
• Tests updated to lock behavior.

On cleanup: if metric keys are bounded (heap_used/rss/etc.), opt‑in cleanup is sufficient. If keys can grow, I recommend host‑scheduled cleanup (e.g., every 5m, prune >1h idle). I can add a small startAutoCleanup/stopAutoCleanup helper if you prefer.