Flatten stdouttrace Exporter.initSelfObservability into Exporter.New

[MrAlias]

• Do not use side-effect programming to setup an exporter. Make the setup explicit, unique, and local.
• Report instrument creation errors back to the user instead of passing them to otel.Handle.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.9%. Comparing base (2ab7d4d) to head (c43244b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7197   +/-   ##
=====================================
  Coverage   82.9%   82.9%           
=====================================
  Files        264     264           
  Lines      24628   24630    +2     
=====================================
+ Hits       20424   20433    +9     
+ Misses      3821    3817    -4     
+ Partials     383     380    -3     

Files with missing lines	Coverage Δ
exporters/stdout/stdouttrace/trace.go	83.6% <100.0%> (+9.1%)	⬆️

... and 3 files with indirect coverage changes

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