Added: Rails 8.0 support

[marcotc]

What does this PR do?
Adds support for all existing instrumentation for Rails 8.0.x.
The only production change is in how ActiveSupport cache keys are reported in a span (there's some special handling added for Rails 8), but other parts stay identical.

The tricky part was getting CI to run: Rails 8 really doesn't like to be restarted in-memory, like we do for all other versions of Rails. I had to resort to forking on each test run, something that is not actually supported by RSpec, so I added support for it in spec/support/execute_in_fork.rb.

Change log entry
Yes. Add support for Rails 8.0.

Additional Notes:
It was soooo hard to get the tests to run, without making a complete and utter mess in the process.

How to test the change?
All changes have automated tests.

[github-actions]

Thank you for updating Change log entry section 👏

^{Visited at: 2025-03-25 18:39:46 UTC}

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-05-22 16:05:21

Comparing candidate commit 5e75c0c in PR branch rails-8 with baseline commit 1a8ded2 in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 43 metrics, 1 unstable metrics.

scenario:profiling - Allocations ()

• 🟩 throughput [+189853.318op/s; +195509.180op/s] or [+6.027%; +6.206%]

[datadog-datadog-prod-us1]

Datadog Report

Branch report: rails-8
Commit report: 5e75c0c
Test service: dd-trace-rb

✅ 0 Failed, 21686 Passed, 1299 Skipped, 4m 11.87s Total Time

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.19251% with 104 lines in your changes missing coverage. Please review.

Project coverage is 97.62%. Comparing base (1a8ded2) to head (5e75c0c).

Files with missing lines	Patch %	Lines
...ec/datadog/tracing/contrib/rails/support/rails8.rb	10.81%	66 Missing ⚠️
spec/support/execute_in_fork.rb	61.53%	25 Missing ⚠️
...ng/contrib/active_support/cache/instrumentation.rb	27.27%	8 Missing ⚠️
...action_pack/action_dispatch/journey/router_spec.rb	97.69%	3 Missing ⚠️
...og/tracing/contrib/active_support/cache/patcher.rb	50.00%	1 Missing ⚠️
.../datadog/tracing/contrib/rails/support/database.rb	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4455      +/-   ##
==========================================
- Coverage   97.74%   97.62%   -0.12%     
==========================================
  Files        1455     1459       +4     
  Lines       87371    87558     +187     
  Branches     4492     4537      +45     
==========================================
+ Hits        85397    85481      +84     
- Misses       1974     2077     +103     

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