Fix gateway otlp config

[flomonster]

• service_name does not work (we could delete it and use the default env var)
• figment::merge seems to work the other way around
• We should move OTEL_SERVICE_NAME to docker files for all services

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.88%. Comparing base (5d6174c) to head (ad7971d).
Report is 4 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10529      +/-   ##
==========================================
- Coverage   81.88%   81.88%   -0.01%     
==========================================
  Files        1078     1078              
  Lines      107139   107139              
  Branches      724      724              
==========================================
- Hits        87731    87728       -3     
- Misses      19368    19371       +3     
  Partials       40       40              

Flag	Coverage Δ
editoast	74.17% <ø> (-0.01%)	⬇️
front	89.45% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.50% <ø> (ø)
tests	88.14% <ø> (ø)

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[woshilapin]

I've removed the configuration options for service name in gateway and osrdyne: let's use the standard OTEL_SERVICE_NAME environment variable for that. I've then set up that variable directly in the Dockerfile of the three Rust components.

Finally, I've remove the enable in gateway's configuration as it's a leftover from b154bad.

I still need to test more this figment::merge to understand what is going on since it looks really weird.

[bloussou]

LGTM 🚀