Encapsulate stdouttrace.Exporter instrumentation in internal package

[MrAlias]

Follow guidelines and move instrumentation into its own type.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.9%. Comparing base (a07b7e6) to head (5416fc5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7307   +/-   ##
=====================================
  Coverage   82.9%   82.9%           
=====================================
  Files        266     267    +1     
  Lines      24984   24997   +13     
=====================================
+ Hits       20733   20744   +11     
- Misses      3876    3877    +1     
- Partials     375     376    +1     

Files with missing lines	Coverage Δ
...out/stdouttrace/internal/observ/instrumentation.go	100.0% <100.0%> (ø)
exporters/stdout/stdouttrace/trace.go	73.8% <100.0%> (-17.7%)	⬇️

... and 2 files with indirect coverage changes

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[Copilot]

Pull Request Overview

This PR refactors the stdouttrace.Exporter self-observability instrumentation by moving it into an internal package following OpenTelemetry Go project guidelines. The change encapsulates the instrumentation logic into a dedicated type to improve code organization and maintainability.

• Extracted self-observability logic into a new internal/observ package
• Created a dedicated Instrumentation type to handle metrics and measurement
• Removed extensive test cases from the main test file that are now covered by the internal package tests

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
versions.yaml	Added version reference configuration for the stdouttrace module
exporters/stdout/stdouttrace/trace_test.go	Simplified tests by using the new observ package constants and removed detailed test cases
exporters/stdout/stdouttrace/trace.go	Refactored to use the new Instrumentation type from the observ package
exporters/stdout/stdouttrace/internal/version.go	Added version constant for the internal package
exporters/stdout/stdouttrace/internal/observ/instrumentation_test.go	New comprehensive test suite for the instrumentation functionality
exporters/stdout/stdouttrace/internal/observ/instrumentation.go	New package containing the extracted instrumentation logic
.codespellignore	Added "observ" to the spell check ignore list

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[pellared]

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