refactor: improve autoscaler metrics fetcher design

[Jeffwan]

Pull Request Description

• Extract lightweight EngineMetricsFetcher with endpoint-based parameters
• Clean up MetricFetcher interface to remove mixed abstraction levels
• Separate autoscaler business logic from HTTP utility layer

The PR refactor the fetching abstraction:

  MetricClient (autoscaler business logic)
      ↓ uses
  MetricFetcher (autoscaler's metrics interface)
      ↓ delegates to
  EngineMetricsFetcher (pure HTTP utility)

the EngineMetricsFetcher is also shared by router, which fetch engine metrics. Now, they use same library.

Related Issues

Resolves: part of #1422

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[gemini-code-assist]

Summary of Changes

Hello @Jeffwan, 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 significantly refactors the metrics fetching and processing logic within the autoscaler. It introduces a new, centralized EngineMetricsFetcher to streamline the retrieval of typed metrics directly from inference engine pods. This change aims to improve the modularity and maintainability of the codebase by separating the autoscaler's business logic from the low-level HTTP utility layer, leading to more robust and reliable metric collection.

Highlights

• Centralized Engine Metrics Fetching: A new EngineMetricsFetcher is implemented in pkg/metrics/engine_fetcher.go to provide a unified and typed way to fetch metrics from inference engine pods, replacing ad-hoc HTTP calls.
• Improved Abstraction and Decoupling: The MetricFetcher interface and its implementations (especially RestMetricsFetcher) are refactored to delegate metric retrieval to the new EngineMetricsFetcher, cleaning up the autoscaler's core logic.
• Enhanced Metric Processing in Cache: The pkg/cache/cache_metrics.go now leverages the EngineMetricsFetcher for updating pod metrics, deprecating older, less structured methods and enabling more comprehensive metric handling.
• Robust Metric Collection and Validation: New validation checks for metric values (e.g., NaN, infinite, out-of-range) are added in pkg/controller/podautoscaler/metrics/utils.go, along with improved error reporting for partial metric collection failures, increasing the system's resilience.

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[gemini-code-assist]

Code Review

This pull request refactors the metrics fetching mechanism for the autoscaler, introducing a centralized EngineMetricsFetcher. The changes are a significant improvement, separating concerns and creating a more robust foundation. My review focuses on further refining the implementation by leveraging standard library functions, improving correctness in metric fetching logic, and reducing code duplication. Key suggestions include using net.SplitHostPort for endpoint parsing, resolving an issue with hardcoded engine types, and centralizing helper functions.

[Jeffwan]

/cc @googs1025 please help review this change. I plan to do more refactor on autoscaler to unblock #1260