benchmark: use custom duration in prom metric

[arkodg]

• determine the time duration more accurately instead of using 30s, which may be reading values across tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.05%. Comparing base (c19352e) to head (c672508).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6811      +/-   ##
==========================================
+ Coverage   71.04%   71.05%   +0.01%     
==========================================
  Files         225      225              
  Lines       39810    39810              
==========================================
+ Hits        28282    28288       +6     
+ Misses       9859     9857       -2     
+ Partials     1669     1665       -4     

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[zhaohuabing]

LGTM thanks!

[zirain]

/retest