brpc 代码中定义的一些bvar监控项比如process_cpu_usage一直不更新

**Describe the bug (描述bug)**
brpc 代码中定义的一些bvar监控项一直不更新。 比如process_cpu_usage，process_memory_data_and_stack等。
但是有一些自定义的bvar是变动的。
但是进程重启后可能又恢复， 可能再次重启又不动了。

**To Reproduce (复现方法)**

**Expected behavior (期望行为)**

**Versions (各种版本)**
OS: 5.4.0-139-generic #156-Ubuntu SMP Fri Ja
Compiler:
brpc: 1.7.0
protobuf:

**Additional context/screenshots (更多上下文/截图)**
这个就是一直不变，一直是0

No source file, test, or reproduction steps are named. Start by tracing the definitions and update paths for process_cpu_usage and process_memory_data_and_stack, comparing them with a custom bvar that changes, then reproduce on brpc 1.7.0 and the stated Ubuntu version. Done means the built-in monitoring values update reliably across process restarts and the failure has regression coverage.

Written by the indexing model from the issue text.