bvar::MultiDimension::delete_stats() may cause core dump

[ketor]

Describe the bug (描述bug)
There is a bug in function MultiDimension::delete_stats().
This function use a FlatMap ( MetricMap tmp_map ) and declare a lambda function clear_fn to do clear data in the DoublyBufferedData _metric_map.

But the FlatMap ( MetricMap tmp_map ) is not initialized by init() before use it to do swap().

This will cause segment fault if call get_stats() after delete_stats().

To Reproduce (复现方法)
Implement a new MulitDimension object and call get_stats() in one thread, while another thread do delete_stats() at the same time.
The DoublyBufferedData data structure is thread-safe, but brpc use a wrong method to do delete_stats() which will cause segment fault here.

Expected behavior (期望行为)
No segment fault.

Versions (各种版本)
OS: Ubuntu 22.04/ Rocky 8.x
Compiler:gcc 11
brpc:master
protobuf:3.20.3

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