feat(data-pipeline): add span links FFI

[lloeki]

AI-generated code disclosure: this PR was implemented with substantial AI assistance and manually reviewed and validated.

What does this PR do?

Adds an atomic batched FFI for complete span links, including 128-bit trace IDs, span IDs, flags, tracestate, attributes, and ordering. All borrowed input is validated and copied before insertion.

Motivation

The Rust trace model supports span links, but native exporter consumers cannot populate it. This fills the span-link portion of the payload parity work tracked by APMSP-3219.

Additional Notes

dropped_attributes_count is intentionally deferred by additive commit 961044ea. Neither the Agent v0.4 nor indexed V1 span-link schema defines the field, and current Datadog tracers do not produce non-zero counts. The original implementation remains in the preceding commit and can be restored by reverting the deferral commit.

Stack position: L2, current PR: DataDog/libdatadog PR 2305.

These open L0-L5 changes are not included in libdatadog v38.0.0. They are planned to ship together in a single libdatadog v39 release, with the common dd-trace-rb uptake tracked by R3 DataDog/dd-trace-rb PR 6143.

Complete expected libdatadog review and merge order for that single v39 release:

Level	libdatadog PR	Matching dd-trace-rb consumer
L0	DataDog/libdatadog PR 2302	Release prerequisite; no payload API consumer
L1	DataDog/libdatadog PR 2303	R4 DataDog/dd-trace-rb PR 6130
L2 (current)	DataDog/libdatadog PR 2305	R5 DataDog/dd-trace-rb PR 6129
L3	DataDog/libdatadog PR 2301	R6 DataDog/dd-trace-rb PR 6134
L4	DataDog/libdatadog PR 2304	R7 DataDog/dd-trace-rb PR 6132
L5	DataDog/libdatadog PR 2300	R8 DataDog/dd-trace-rb PR 6133

L0-L3 are independent and can be reviewed in parallel. Merging them in the listed order is still recommended because it reduces integration and release-branch conflicts, and lands panic containment before the new payload surfaces. L4 is stacked on L1, and L5 is stacked on L4.

The matching payload consumers are R4 DataDog/dd-trace-rb PR 6130, R5 DataDog/dd-trace-rb PR 6129, R6 DataDog/dd-trace-rb PR 6134, R7 DataDog/dd-trace-rb PR 6132, and R8 DataDog/dd-trace-rb PR 6133. All depend through the common v39 uptake in R3 DataDog/dd-trace-rb PR 6143. The downstream review and merge order after R3 is R4, R5, R6, R7, then R8; R7 also depends on R4, and R8 depends on R7.

DataDog/libdatadog-rb v37 PR 65 and DataDog/libdatadog-rb v38 PR 66 are separate ordered prerequisite releases only for R2 DataDog/dd-trace-rb PR 6131 and its ForkSafeRuntime uptake. They do not contain the open L0-L5 changes or payload APIs. R3 is stacked on R2, but R3, rather than either prerequisite package, is the common v39 payload-API uptake.

L0 tracks APMSP-3830; L1-L5 implement the payload work tracked by APMSP-3219.

How to test the change?

Prior local validation completed successfully:

• nix develop -c cargo nextest run -p libdd-data-pipeline-ffi passed 59 tests, including atomic invalid-UTF-8 failure, ordering, and ownership coverage.
• nix develop -c cargo test -p libdd-trace-utils passed the unit, integration, and doc-test coverage for all affected encoders and decoders.
• nix develop -c cargo clippy -p libdd-trace-utils -p libdd-data-pipeline-ffi --all-targets -- -D warnings passed.
• nix develop .#nightly --command cargo fmt --all -- --check passed.
• nix develop --command bundle exec rspec spec/datadog/tracing/transport/native/tracer_span_spec.rb spec/datadog/tracing/transport/native/transport_spec.rb spec/datadog/tracing/transport/native/conformance_spec.rb passed 119 native transport examples against the locally built artifact.
• nix develop --command bundle exec rspec spec/datadog/tracing/span_link_spec.rb spec/datadog/tracing/distributed/propagation_spec.rb passed 69 distributed-propagation examples with 1 existing pending example.
• nix develop --command env BUNDLE_GEMFILE=gemfiles/ruby_4.0_opentelemetry.gemfile bundle exec rspec spec/datadog/opentelemetry_spec.rb passed 164 OpenTelemetry examples.
• Full Standard and Steep checks passed on the matching consumer branch.

[datadog-prod-us1-3]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 99.22%
• Overall Coverage: 75.13% (+0.06%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: f21ea81 | Docs | Datadog PR Page | Give us feedback!}

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 0bffa493ac

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[pr-commenter]

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[lloeki]

Draft dd-trace-rb example consumer: DataDog/dd-trace-rb#6129.

[lloeki]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: 961044ea58

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[hoolioh]

/merge

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-08-05 13:16:05 UTC ℹ️ Start processing command /merge

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2026-08-05 14:04:23 UTC ℹ️ MergeQueue: merge request added to the queue

The expected merge time in main is approximately 53m (p90).

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