[BUG] sentinel dashboard监控仪表盘出现监控掉底

[worker24h]

Issue Description

Type: bug report

Describe what happened

如图所示：dashboard中监控显示，出现掉底，但实际metrics日志中存在数据

Describe what you expected to happen

修复监控掉底场景

How to reproduce it (as minimally and precisely as possible)

1. springboot应用，引入官方starter
2. 发压端流量在500左右即可
3. 代码调用，SphU.entry(resourceName, EntryType.IN, 1)

Tell us your environment

Anything else we need to know?

[worker24h]

通过查看http response，发现丢失一个数据

[LearningGp]

本地metrics文件没有丢失这个点话，就可能是com.alibaba.csp.sentinel.node.metric.MetricsReader#readMetricsByEndTime的问题了，可以用arthas watch 下看看