[Bug] KVCache: fix metrics

[DwyaneShi]

Pull Request Description

[Please provide a clear and concise description of your changes here]

Related Issues

Resolves: #[Insert issue number(s)]

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[gemini-code-assist]

Summary of Changes

Hello @DwyaneShi, 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 addresses a bug in the KVCache metrics system by refactoring how metric attributes are initialized. Previously, some metric counters and lists were initialized as class-level defaults, which could lead to incorrect state management. The changes move these initializations into the respective class constructors, ensuring that each instance of OpMetrics, UsageMetrics, and BaseCacheMetrics starts with a clean and correct state for its metrics, thereby improving the accuracy of metric collection.

Highlights

• Metric Initialization Refactor: Moved the initialization of various metric counters and lists (e.g., num_ops, num_errors, num_prefixes, num_tokens, num_fetched_tokens, capacity_nbytes, used_nbytes, num_tokens_hit, num_tokens_miss) from class-level default assignments to instance-level initialization within the __init__ methods of OpMetrics, UsageMetrics, and BaseCacheMetrics.
• Bug Fix: This change specifically fixes a bug where metric values might not have been correctly reset or managed across different instances or uses, ensuring accurate metric collection for KVCache operations.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a critical issue with metric tracking by moving the initialization of metric attributes from class-level to instance-level within the __init__ methods. This prevents state from being shared across different metric instances, ensuring accurate and independent metric collection for operations like 'put', 'get', and 'exists'. The changes are applied consistently across OpMetrics, UsageMetrics, and BaseCacheMetrics.

I've identified a couple of related inconsistencies in the reset() logic for metrics which I've detailed in my comments. Addressing these would further improve the correctness and consistency of the metrics system.