Fix: Etcd metrics not correct

[little-cui]

背景描述：metrics是基于事件驱动计数的，但由于缓存增加了定时重置机制，如重置时刚好发生了实例变化，这样会导致metrics计算错误
修复方案：重构metrics机制为定期计算。

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[tianxiaoliang]

记录修复思路

[tianxiaoliang]

这个周期是多久一次，要可以配置，否则后续由于压力导致需要调整时，可能不会很方便

[little-cui]

可以配置的

metrics:
  interval: 30s