Add Ruby 3.4.0-preview2 support for CircleCI tests

[sarahchen6]

What does this PR do?
This PR adds "3.4" versions to the config.yml file under .circleci everywhere there is a comment to # ADD NEW RUBIES HERE.

Motivation:
Though the official Ruby 3.4 is not yet released, we can temporarily add Ruby 3.4.0-preview2. This allows us to start testing with Ruby 3.4 in our CI and spot -> address issues early. After Ruby 3.4 is officially released post-Dec 25, 2024, we can replace the preview with it.

Change log entry
No change log entry for now. Thinking to have a "Ruby 3.4.0-preview2 support added" update after it is fully supported.

Additional Notes:
This PR is one step toward full support and only addresses CircleCI workflow tests. It acts alongside #4038 and #4039 .

How to test the change?
This change can be tested in CI.

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-11-08 21:02:17

Comparing candidate commit f1c7fe4 in PR branch sarahchen6/add-ruby-3.4-circleci with baseline commit c8bd1e4 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 29 metrics, 2 unstable metrics.

[p-datadog]

Rack::Handler was also deleted in 3.1.0. The removal is mentioned in https://github.com/rack/rack/blob/main/UPGRADE-GUIDE.md but it looks like some/all of those changes were made in 3.1.0 and not in 3.0 as advertised.

Could be fixed by requiring rackup in the affected test?

[p-datadog]

The following makes the test pass on ruby-3.4 config:

diff --git a/spec/datadog/tracing/contrib/suite/integration_spec.rb b/spec/datadog/tracing/contrib/suite/integration_spec.rb
index eaf37eb1f..eaa9ab0ba 100644
--- a/spec/datadog/tracing/contrib/suite/integration_spec.rb
+++ b/spec/datadog/tracing/contrib/suite/integration_spec.rb
@@ -8,7 +8,7 @@ require 'semantic_logger'
 
 require 'rack'
 # `Rack::Handler::WEBrick` was extracted to the `rackup` gem in Rack 3.0
-require 'rackup' if Gem::Version.new(Rack::RELEASE) >= Gem::Version.new('3')
+require 'rackup/handler/webrick' if Gem::Version.new(Rack::RELEASE) >= Gem::Version.new('3')
 require 'webrick'
 
 RSpec.describe 'contrib integration testing', :integration do
@@ -164,7 +164,7 @@ RSpec.describe 'contrib integration testing', :integration do
             end
           end.to_app
 
-          server.mount '/', Rack::Handler::WEBrick, app
+          server.mount '/', Rackup::Handler::WEBrick, app
 
           @thread = Thread.new { server.start }
           try_wait_until { started }

I haven't checked if this would also work on rack 3.0, and I am quite certain this will NOT work on rack 2 and lower therefore the change needs to be conditional on rack version.

[ivoanjo]

Slight suggestion: This PR is getting so big at this point that GitHub literally does not show me the complete list of files changed when I go on the diff tab.

May be worth splitting some of the changes into smaller PRs that could go in already? E.g. maybe a separate PR for the ffi/webrick stuff, that doesn't even add 3.4 yet, etc?

[sarahchen6]

This PR was split up. See #4114 .