feat(api-service): add experiment-level resource admission control

[JacksonMei]

Implement three-tier admission (P0/P1/P2) with watermark-based throttling for cluster resource protection:

• P0: known experiments always admitted
• P1: new experiments gated by cluster utilization watermark and reserved capacity check
• P2: requests missing required labels rejected (400)

Includes Prometheus metrics, sliding-window peak tracking, response-capture middleware for instance-experiment mapping, and fallback record-only cleanup for unreachable nodes.

[gemini-code-assist]

Summary of Changes

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This pull request introduces a robust experiment-level resource admission control system to the API service. The primary goal is to enhance cluster stability and resource fairness by intelligently managing the creation of new environment instances. It prioritizes existing, known experiments, while new experiments are admitted only if cluster utilization is below a configurable watermark and sufficient capacity is reserved. Additionally, it enforces mandatory labels for requests and provides comprehensive metrics for monitoring the system's behavior, ensuring a more resilient and predictable environment for running experiments.

Highlights

• Three-Tier Admission Control: Implemented a three-tier admission control system (P0/P1/P2) to manage resource allocation for experiments.
• Watermark-Based Throttling: Introduced watermark-based throttling to gate new experiments (P1) based on cluster utilization and reserved capacity.
• Required Label Enforcement: Requests missing required labels (P2) are now rejected with a 400 error, ensuring metadata consistency.
• Observability and Metrics: Integrated Prometheus metrics for admission decisions, reserved capacity, experiment counts, and cluster utilization.
• Fallback Cleanup Mechanism: Added a fallback record-only cleanup mechanism for instances on unreachable nodes, preventing resource leaks.

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