editoast: add telemetry to core requests

[woshilapin]

We don't have any information about how long it takes for each request to core. This telemetry should give us some information about it.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.34%. Comparing base (725b89a) to head (5989d4b).
Report is 1 commits behind head on dev.

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #7958      +/-   ##
============================================
- Coverage     28.35%   28.34%   -0.01%     
  Complexity     2075     2075              
============================================
  Files          1270     1270              
  Lines        155669   155626      -43     
  Branches       3076     3076              
============================================
- Hits          44135    44112      -23     
+ Misses       109701   109681      -20     
  Partials       1833     1833              

Flag	Coverage Δ
core	75.02% <ø> (ø)
editoast	71.30% <100.00%> (+0.02%)	⬆️
front	9.95% <ø> (ø)
gateway	2.34% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	72.93% <ø> (ø)

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[flomonster]

LGTM. (Not tested)