Activate _dd.p.ksr tests for all languages

[bm1549]

Motivation

Activate the Test_Knuth_Sample_Rate (_dd.p.ksr) parametric tests across all tracer languages. Previously, most languages had these tests partially activated via the "easy win" script with individual sub-test exclusions. This PR cleans up those manifest entries and updates the tests themselves to be more thorough and correct.

Changes

Manifest updates (all 9 languages)

• C++: Removed per-sub-test missing_feature exclusion, bumped version gate from >=2.0.0 to >=2.1.0. Cleaned up "easy win activation script" comments.
• .NET: Removed per-sub-test missing_feature exclusion, bumped version gate from >=3.36.0 to >=3.41.0. Cleaned up "easy win activation script" comments.
• Go: Removed per-sub-test missing_feature exclusion, bumped version gate from >=2.5.0 to >=2.8.0. Cleaned up "easy win activation script" comments.
• Java: Removed per-sub-test missing_feature exclusion, bumped version gate from >=1.58.2+... to >=1.61.0. Cleaned up "easy win activation script" comments.
• Node.js: Changed from missing_feature to version-gated (>=5.93.0).
• PHP: Changed from missing_feature to version-gated (>=1.18.0).
• Python: Bumped version gate from v3.14.0.dev to v4.7.0.
• Ruby: Removed per-sub-test missing_feature exclusion, bumped version gate from >=2.27.0 to >=2.31.0. Cleaned up "easy win activation script" comments.
• Rust: Replaced per-sub-test missing_feature exclusion with a class-level version gate (>=0.4.0).

Test changes (test_sampling_span_tags.py)

• test_sampling_knuth_sample_rate_trace_sampling_rule: Simplified from a 3-trace/3-flush pattern to a single span, using find_only_span. Added precision boundary test cases (6-digit precision cutoff, rounding to zero, rounding up with 0.00000051). Renamed parameter from sample_rate to expected_ksr for clarity. Added DD_TRACE_STATS_COMPUTATION_ENABLED=false to all cases — required for Go, which defaults this to true and will otherwise drop p0 traces client-side before they reach the test agent (see below).
• test_sampling_extract_knuth_sample_rate_distributed_tracing_datadog: Added a second parametrized case that verifies upstream _dd.p.ksr is propagated unchanged even when local sampling rules are configured.
• test_sampling_knuth_sample_rate_not_set_for_default (new): Verifies that when no sampling rules or agent rates are configured, _dd.p.ksr is either absent or "1".

Why DD_TRACE_STATS_COMPUTATION_ENABLED=false is needed for Go

dd-trace-go defaults DD_TRACE_STATS_COMPUTATION_ENABLED=true. The test agent advertises client_drop_p0s:true and /v0.6/stats, so Go's canDropP0s() returns true. When a sampling rule sets UserReject priority (which happens ~100% of the time at rates like 0.000001), Go drops the trace client-side before transport — the test agent receives 0 traces.

Other tracers are unaffected by this env var:

• Python: _compute_stats_enabled defaults to False
• Node.js: no client-side p0 dropping mechanism
• Java: dropping disabled because test agent's "version":"test" fails AgentVersion.isVersionBelow(), so supportsClientSideStats=false

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed and the CI green, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

manifests/cpp.yml                                                       @DataDog/dd-trace-cpp
manifests/dotnet.yml                                                    @DataDog/apm-dotnet @DataDog/asm-dotnet
manifests/golang.yml                                                    @DataDog/dd-trace-go-guild
manifests/java.yml                                                      @DataDog/asm-java @DataDog/apm-java
manifests/nodejs.yml                                                    @DataDog/dd-trace-js
manifests/php.yml                                                       @DataDog/apm-php @DataDog/asm-php
manifests/python.yml                                                    @DataDog/apm-python @DataDog/asm-python
manifests/ruby.yml                                                      @DataDog/ruby-guild @DataDog/asm-ruby
manifests/rust.yml                                                      @DataDog/apm-rust
tests/parametric/test_sampling_span_tags.py                             @DataDog/system-tests-core @DataDog/apm-sdk-capabilities

[datadog-prod-us1-6]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
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[mabdinur]

We need to address the dotnet and cpp failures but aside from that the changes look good to me

[bm1549]

The golang dev test is currently failing, but the changes have been merged to main in DataDog/dd-trace-go#4603

I expect that test will start passing when the dev artifact updates