[ISSUE #8935] Fix behind metrics unit error in timer message store

[GenerousMan]

Which Issue(s) This PR Fixes

Fixes #8935

Brief Description

How Did You Test This Change?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 47.58%. Comparing base (797024f) to head (5bd2381).

Files with missing lines	Patch %	Lines
...apache/rocketmq/store/timer/TimerMessageStore.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #8936      +/-   ##
=============================================
+ Coverage      47.55%   47.58%   +0.03%     
- Complexity     11733    11742       +9     
=============================================
  Files           1304     1304              
  Lines          91058    91058              
  Branches       11675    11675              
=============================================
+ Hits           43303    43331      +28     
+ Misses         42405    42386      -19     
+ Partials        5350     5341       -9     

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[guyinyou]

lgtm