core: stdcm: add some telemetry spans

[eckter]

Some extra spans, especially for the request reading/parsing steps

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.78%. Comparing base (74a2bcb) to head (c500060).
Report is 15 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10482      +/-   ##
==========================================
- Coverage   81.78%   81.78%   -0.01%     
==========================================
  Files        1073     1073              
  Lines      106580   106578       -2     
  Branches      731      731              
==========================================
- Hits        87166    87161       -5     
- Misses      19375    19378       +3     
  Partials       39       39              

Flag	Coverage Δ
editoast	74.21% <ø> (-0.02%)	⬇️
front	89.34% <ø> (-0.01%)	⬇️
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.50% <ø> (ø)
tests	87.05% <ø> (ø)

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