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

[UndefinedSy]

What type of PR is this?

• 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:

• 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_.