DEBUG-3182 DI railtie

[p-datadog]

What does this PR do?

Adds a Railtie for dynamic instrumentation. The railtie delays loading of DI contrib code (currently, ActiveRecord serializer) until Rails components have been loaded.

Also adds an integration test verifying AR serialization works correctly.

Motivation:

Improving DI reliability

Change log entry

Yes: improve loading of dynamic instrumentation components

Additional Notes:

How to test the change?

An integration test is added in this PR

[datadog-datadog-prod-us1]

Datadog Report

Branch report: di-railtie
Commit report: c050627
Test service: dd-trace-rb

❌ 1 Failed (0 Known Flaky), 22129 Passed, 1476 Skipped, 5m 7.3s Total Time

❌ Failed Tests (1)

• LOADED variable - rspec - Details

  Expand for error

   uninitialized constant Datadog::DI::Contrib
   
   Failure/Error: Datadog::DI::Contrib.load_now_or_later
   
   NameError:
     uninitialized constant Datadog::DI::Contrib
   ./lib/datadog/auto_instrument.rb:9:in \`<main>'
   ./spec/datadog/auto_instrument_spec.rb:50:in \`block in auto_instrument'
   ./spec/datadog/auto_instrument_spec.rb:52:in \`block in <main>'
   ./spec/datadog/tracing/contrib/support/tracer_helpers.rb:96:in \`block in TracerHelpers'
   ...
  

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-10 22:50:51

Comparing candidate commit c050627 in PR branch di-railtie with baseline commit c856145 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.