fix. stats manager may cause crash when stats with new labels.

[UndefinedSy]

What type of PR is this?

• [x] bug
• [ ] feature
• [ ] enhancement

What problem(s) does this PR solve?

Issue(s) number:

5932

Description:

when continuously adding new labels to statsManager's nameMap, one may crash on using the previously returned iterator from nameMap.

How do you solve it?

using read lock to protect the iterator returned from sm.nameMap_, which is a std::unordered_map.

Special notes for your reviewer, ex. impact of this fix, design document, etc:

process will not crash on using statsManager.

Checklist:

Tests:

• [x] Unit test(positive and negative cases)
• [ ] Function test
• [ ] Performance test
• [ ] N/A

Affects:

• [ ] Documentation affected (Please add the label if documentation needs to be modified.)
• [ ] Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• [ ] If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• [ ] Performance impacted: Consumes more CPU/Memory

Release notes:

Fixed the bug that process crashes on concurrent accessing stats manager's nameMap_.