使用 gdb_bthread_stack.py 时报错

[wqshr12345]

Describe the bug
我目前使用了 brpc 1.14.1 release 版本。在调试一个线上程序时，我尝试使用 gdb_bthread_stack.py 用来查看运行时堆栈，但是我遇到了如下错误：
Python Exception <class 'gdb.error'> There is no member or method named _agents.:
Error occurred in Python command: There is no member or method named _agents.

To Reproduce
gdb
attach pid
source gdb_bthread_stack.py
bthread_start

Expected behavior
gdb_bthread_stack.py 可以正常运作，查看 bthread 堆栈

Versions
OS: Debian 10
Compiler: gcc8
brpc: 1.14.1
protobuf:

Additional context/screenshots

[TousakaRin]

fixed