chore: sdk/internal/x - generate x package from shared template

[nikhilmantri0902]

Closes #7384

• Add sdk/internal/gen.go with gotmpl directives to generate x/x.go and x/x_test.go
• Refactor sdk/internal/x/x.go to generic Feature[T] with multi-key support and Keys()
• Move Resource feature into sdk/internal/x/features.go (case-insensitive true enables)
• Add tests: generic feature tests in x_test.go and Resource tests in features_test.go
• Aligns with sdk/log/internal/x pattern introduced in PR chore: sdk/log/internal/x - generate x package from x component template  #7389

Internal note: replaces Key() with Keys() for internal Feature API. Build and lints pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.2%. Comparing base (b37822b) to head (de375e5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7495   +/-   ##
=====================================
  Coverage   86.2%   86.2%           
=====================================
  Files        295     294    -1     
  Lines      25864   25844   -20     
=====================================
- Hits       22302   22286   -16     
+ Misses      3189    3185    -4     
  Partials     373     373           

Files with missing lines	Coverage Δ
sdk/internal/x/features.go	100.0% <100.0%> (ø)
sdk/internal/x/x.go	100.0% <100.0%> (ø)
sdk/trace/internal/observ/batch_span_processor.go	100.0% <ø> (ø)
sdk/trace/internal/observ/tracer.go	100.0% <ø> (ø)

... and 2 files with indirect coverage changes

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[flc1125]

Please fix the lint errors; we can verify them with make ci.

[nikhilmantri0902]

@flc1125 done. Please check.

[MrAlias]

This should replace sdk/trace/internal/x instead of being added additionally. There should not be more than one x package in a module to prevent duplication and confusion.

The replacement and associated updates should be included here or things should be kept the way they currently are.

[nikhilmantri0902]

This should replace sdk/trace/internal/x instead of being added additionally. There should not be more than one x package in a module to prevent duplication and confusion.

The replacement and associated updates should be included here or things should be kept the way they currently are.

@flc1125 and @MrAlias should I make changes based on this in this PR or we can take it up later? If for this PR, can you once explain what needs to be done I ll do that

[MrAlias]

Include the changes in this PR. You "should replace sdk/trace/internal/x instead of [just adding this new package]. There should not be more than one x package in a module to prevent duplication and confusion."

Any dependencies on sdk/trace/internal/x need to be updated to use this added package instead.

[nikhilmantri0902]

@flc1125 @MrAlias I replaced the usages of sdk/trace/internal/x.

Then I thought why would I even need to keep this package. Should I delete this package sdk/trace/internal/x? I tried removing everything, but make ci regenerates the code. Should we just keep those files for now. In the previous commit, they got regenerated when I ran make ci.

[flc1125]

Should I delete this package sdk/trace/internal/x?

We can remove them.