Fix:Rails Runner instrumentation

[marcotc]

What does this PR do?
The Rails Runner instrumentation was being loaded too late, effectively not being able to create its spans.

Change log entry
Yes. The Rails Runner instrumentation has been fixed, and should now create Rails Runner spans.

How to test the change?
All changes have tests. And this time, they reflect a real Rails setup.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: fix-runner
Commit report: 1fd6d88
Test service: dd-trace-rb

❌ 1 Failed (0 Known Flaky), 20935 Passed, 1298 Skipped, 4m 14.73s Total Time

❌ Failed Tests (1)

• Datadog::Tracing::Contrib::Rails::Runner with a file path with an error reading the source file creates an error span - rspec - Details

  Expand for error

   expected LoadError but nothing was raised
   
   Failure/Error: expect { run }.to raise_error(LoadError)
     expected LoadError but nothing was raised
   ./spec/datadog/tracing/contrib/rails/runner_spec.rb:102:in 'block (4 levels) in <top (required)>'
   ./spec/datadog/tracing/contrib/rails/runner_spec.rb:67:in 'block (4 levels) in <top (required)>'
   ./spec/datadog/tracing/contrib/rails/runner_spec.rb:62:in 'block (3 levels) in <top (required)>'
   ./spec/datadog/tracing/contrib/rails/support/base.rb:65:in 'block (3 levels) in <top (required)>'
   /usr/local/bundle/gems/climate_control-1.2.0/lib/climate_control.rb:24:in 'block in ClimateControl#modify'
   /usr/local/bundle/gems/climate_control-1.2.0/lib/climate_control.rb:15:in 'Monitor#synchronize'
   ...
  

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-05-27 22:53:57

Comparing candidate commit 1fd6d88 in PR branch fix-runner with baseline commit ff8ae5b in branch master.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 42 metrics, 2 unstable metrics.

scenario:error - error tracking with http - third_party only

• 🟥 throughput [-106.808op/s; -97.339op/s] or [-97.438%; -88.799%]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 97.64%. Comparing base (ff8ae5b) to head (d6820b1).

Files with missing lines	Patch %	Lines
lib/datadog/tracing/contrib/rails/runner.rb	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4681      +/-   ##
==========================================
- Coverage   97.64%   97.64%   -0.01%     
==========================================
  Files        1470     1470              
  Lines       87685    87683       -2     
  Branches     4544     4540       -4     
==========================================
- Hits        85624    85615       -9     
- Misses       2061     2068       +7     

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[Strech]

Awesome 👏🏼