[docs] Document avoiding embedded structs in config structures

[ANAMASGARD]

Description

Add guidelines to coding-guidelines.md explaining why embedded structs should be avoided in configuration definitions. This helps prevent unmarshal issues, naming conflicts, and improves code clarity.

Key points added to the documentation:

• Unmarshal Compatibility: Embedded structs can break custom Unmarshal implementations
• Naming Conflicts: Embedded structs can cause field name collisions when multiple embedded types have fields with the same name
• Clarity: Named fields make the configuration structure more explicit and easier to understand
• Example code: Shows the difference between BAD (embedded) and GOOD (named fields) patterns

Link to tracking issue

Fixes #12719

Testing

Documentation only - no code changes. No testing required.

Documentation

Added new subsection "Avoid Embedded Structs" under the existing "Configuration structs" section in docs/coding-guidelines.md. The section includes:

• Clear explanation of what embedded structs are
• Three key reasons to avoid them in configurations
• Code example demonstrating the preferred approach using named fields with mapstructure tags

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[ANAMASGARD]

@dmathieu I'm not aware of a linter that specifically checks for embedded structs in config files. This could be a good enhancement for the future as a custom linter rule. For now, the documentation should help maintain awareness among contributors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.79%. Comparing base (f724e49) to head (8fadf87).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14340      +/-   ##
==========================================
- Coverage   91.81%   91.79%   -0.03%     
==========================================
  Files         679      679              
  Lines       42857    42857              
==========================================
- Hits        39351    39340      -11     
- Misses       2439     2446       +7     
- Partials     1067     1071       +4     

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[dmathieu]

You don't need to re-request approvals for every merge with main.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[otelbot]

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