[sdk-metrics] Update Exemplars before MetricPoint CompleteUpdate fires

[CodeBlanch]

Changes

• Switches the order of UpdateExemplar & CompleteUpdate inside MetricPoint. According to @utpilla this is required for the delta MetricPoint reclaim feature to function correctly with exemplars enabled.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.71%. Comparing base (6250307) to head (f5c1600).
Report is 230 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5630      +/-   ##
==========================================
+ Coverage   83.38%   85.71%   +2.33%     
==========================================
  Files         297      254      -43     
  Lines       12531    11036    -1495     
==========================================
- Hits        10449     9460     -989     
+ Misses       2082     1576     -506     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.46% <100.00%> (?)
unittests-Solution-Stable	85.68% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/OpenTelemetry/Metrics/MetricPoint.cs	94.24% <100.00%> (+25.76%)	⬆️

... and 113 files with indirect coverage changes