fix(MetricsEventBus): fix some bugs

[PengJingzhao]

What is the purpose of the change?

to solve issue #15295

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 60.77%. Comparing base (6a69ad4) to head (e6df842).
Report is 5 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
...rg/apache/dubbo/metrics/event/MetricsEventBus.java	44.44%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##                3.3   #15296   +/-   ##
=========================================
  Coverage     60.77%   60.77%           
- Complexity    10896    10919   +23     
=========================================
  Files          1885     1886    +1     
  Lines         86087    86128   +41     
  Branches      12896    12903    +7     
=========================================
+ Hits          52317    52342   +25     
- Misses        28321    28324    +3     
- Partials       5449     5462   +13     

Flag	Coverage Δ
integration-tests	33.27% <44.44%> (+0.18%)	⬆️
samples-tests	29.33% <44.44%> (+0.12%)	⬆️
unit-tests	58.90% <44.44%> (+<0.01%)	⬆️

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