[loader] Add 'library_entrypoint.start' metric

[iamluc]

Description

https://datadoghq.atlassian.net/browse/INPLAT-272

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.55%. Comparing base (dc9911b) to head (2d5c2cf).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3009      +/-   ##
============================================
+ Coverage     72.74%   74.55%   +1.80%     
  Complexity     2750     2750              
============================================
  Files           138      111      -27     
  Lines         15060    10900    -4160     
  Branches       1026        0    -1026     
============================================
- Hits          10955     8126    -2829     
+ Misses         3546     2774     -772     
+ Partials        559        0     -559     

Flag	Coverage Δ
appsec-extension	?
tracer-php	74.55% <ø> (-0.02%)	⬇️

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see 28 files with indirect coverage changes

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