feat: Make metrics cache sizes configurable via environment variables

[Copilot]

Hardcoded cache sizes (500, 5000) in src/metrics.py are now configurable.

Changes

• src/config.py: Add MetricsConfig class with three cache size parameters
• src/metrics.py: Replace hardcoded values with config lookups
• env.example: Document new environment variables

New Environment Variables

Variable	Default	Description
METRICS_RESPONSE_TIMES_CACHE_SIZE	500	API response times cache
METRICS_DOC_METRICS_CACHE_SIZE	5000	Document metrics cache
METRICS_ALERTS_CACHE_SIZE	500	Alerts cache

Usage

# Before: hardcoded
if len(metrics.response_times) > 1000:
    metrics.response_times = metrics.response_times[-500:]

# After: configurable
cache_size = get_config().metrics.response_times_cache_size
if len(metrics.response_times) > cache_size * 2:
    metrics.response_times = metrics.response_times[-cache_size:]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Metrics cache sizes hardcoded</issue_title>
<issue_description>## 问题描述
缓存大小硬编码。

受影响的文件

• src/metrics.py 行 112, 132, 178

硬编码的缓存大小

• 500
• 5000

解决方案

应该配置以下环境变量：

• METRICS_RESPONSE_TIMES_CACHE_SIZE 等</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Metrics cache sizes hardcoded #22

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