Purpose

This PR fixes fundamental architectural issues in the Top N Metrics Dashboard implementation that were identified during CI analysis. The previous implementation had critical design flaws that prevented it from working correctly.

Changes

1. Fixed REST Handler inheritance - Now properly extends AbstractRestHandler instead of using incorrect base class
2. Fixed MessageHeaders implementation - Now implements RuntimeMessageHeaders with correct method signatures (getRequestClass, getResponseClass, getResponseStatusCode, getHttpMethod)
3. Fixed MetricStore access - Using public APIs:
   • metricStore.getRepresentativeAttempts() to get job tasks
   • taskMetricStore.getAllSubtaskMetricStores() to get subtasks
   • Instead of attempting to access private members (JobMetricStore, TaskMetricStore.subtasks)
4. Fixed HTTP method references - Using HttpMethodWrapper instead of non-existent HttpMethod
5. Added proper logging - Added Logger instance for better error tracking
6. Added handler registration - Registered TopNMetricsHandler in WebMonitorEndpoint
7. Moved response body to correct package - Moved from legacy.messages to proper job.metrics package

Implementation Details

The implementation now follows Flink's standard REST API architecture pattern:

• Extends AbstractRestHandler<RestfulGateway, EmptyRequestBody, TopNMetricsResponseBody, TopNMetricsMessageParameters>
• Implements proper request handling with MetricFetcher integration
• Uses public MetricStore APIs to safely access metrics data
• Returns Top N metrics for:
  • CPU consumers (Top 5)
  • Backpressured operators (Top 5)
  • GC-intensive tasks (Top 5)

Verifying this change

• Code compiles successfully (excluding unrelated upstream compilation issues)
• Follows Flink REST API architecture patterns
• Uses proper public APIs for MetricStore access
• Code formatted with Spotless
• Integration tests (to be added)

Documentation

This adds a new REST endpoint: GET /jobs/:jobid/metrics/top-n that returns Top N metrics for a job.

Notes

The previous PR #27771 was closed due to fundamental architectural issues. This implementation addresses all identified issues and follows Flink's standard patterns.