Aggregate time initiator

[fraillt]

Changes

Calculation of start_time and time on aggregation is copy/pasted for all aggregators (5 of them).
This PR provides time calculation in a single place.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.4%. Comparing base (9aeae0f) to head (06a5c5b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...-sdk/src/metrics/internal/exponential_histogram.rs	63.6%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2423     +/-   ##
=======================================
- Coverage   79.4%   79.4%   -0.1%     
=======================================
  Files        122     122             
  Lines      21708   21691     -17     
=======================================
- Hits       17255   17238     -17     
  Misses      4453    4453             

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