quantile 格式不一致

**Describe the bug (描述bug)**
目前 mbvar 中的 quantile 和 prometheus_metrics_service.cpp 中对内部数据上报的 quantile 不一致，一个是 90,99,999 一个是 0.9,0.99,0.999

**To Reproduce (复现方法)**

**Expected behavior (期望行为)**
是否统一格式更好一些

**Versions (各种版本)**
OS:
Compiler:
brpc:
protobuf:

**Additional context/screenshots (更多上下文/截图)**

Compare the quantile representation used by mbvar with the internal metrics reporting in prometheus_metrics_service.cpp. First determine which convention the project should use, then make the two sources consistent and verify that the reported quantile values match the chosen format.

Written by the indexing model from the issue text.