[feat][observability] add is_system field to FilterField and FieldMeta

[cuichen01]

Add optional bool is_system to ListSpans FilterField and GetTracesMetaInfo FieldMeta, allowing frontend to distinguish system fields and pass the is_system condition in trace query filters.

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Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ules/observability/application/convertor/filter.go	0.00%	5 Missing and 1 partial ⚠️
.../observability/application/convertor/trace/span.go	0.00%	2 Missing and 2 partials ⚠️
backend/modules/observability/application/trace.go	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #587      +/-   ##
==========================================
+ Coverage   77.46%   77.84%   +0.38%     
==========================================
  Files         679      683       +4     
  Lines       80880    81648     +768     
==========================================
+ Hits        62652    63558     +906     
+ Misses      14493    14375     -118     
+ Partials     3735     3715      -20     

Flag	Coverage Δ
unittests	77.84% <0.00%> (+0.38%)	⬆️

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

Files with missing lines	Coverage Δ
...es/observability/domain/component/config/config.go	50.00% <ø> (ø)
backend/modules/observability/application/trace.go	78.56% <0.00%> (-0.07%)	⬇️
.../observability/application/convertor/trace/span.go	85.42% <0.00%> (-1.41%)	⬇️
...ules/observability/application/convertor/filter.go	39.39% <0.00%> (-3.94%)	⬇️

... and 33 files with indirect coverage changes

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