Large memory footprint: Sentinel MetricNode

### Describe what happened (or what feature you want)

Large memory footprint detected (objects of Sentinel)

线上发布应用后查看堆栈信息发现sentinel相关对象占用太高, 项目中除了测试用例几乎没有埋点信息，请问如何优化以减少内存占用？

详情见图 (heap objects):


### Tell us your environment

Sentinel 1.5.2

Begin with the Sentinel 1.5.2 heap-object evidence in the report and verify the memory footprint in an application with minimal non-test instrumentation. The issue does not identify a file, test, or concrete expected change, so completion would require determining the cause and defining a measurable reduction before implementation.

Written by the indexing model from the issue text.