docs(v4): document experiment instrumentation migration

[marliessophie]

Summary

• Add a dedicated experiment instrumentation subsection to the v3 → v4 migration guide.
• Direct Python and JS/TS users to the Experiment Runner SDK.
• Direct other languages and API integrations to OTEL traces with the experiment attribute propagation guide.
• Explicitly explain that new use of POST /api/public/dataset-run-items is not part of the v4 write path.

Verification

• pnpm run format:check
• pnpm exec node scripts/check-h1-headings.js

Greptile Summary

The PR expands the v3-to-v4 migration guide with dedicated experiment-instrumentation guidance.

• Directs Python and JS/TS users to the Experiment Runner SDK.
• Directs other integrations to one OTEL trace per experiment item and the attribute-propagation guide.
• Clarifies that POST /api/public/dataset-run-items is retained only for legacy compatibility.

Confidence Score: 5/5

The documentation-only change appears safe to merge, with no concrete broken links, rendering problems, or inaccurate migration instructions identified.

The new internal routes and explicit anchors exist, the heading syntax follows established repository conventions, and the migration instructions agree with the detailed SDK and OpenTelemetry experiment documentation.

_{Reviews (1): Last reviewed commit: "docs(v4): document experiment instrument..." | Re-trigger Greptile}

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Project	Deployment	Actions	Updated (UTC)
langfuse-docs	Ready	Preview	Aug 6, 2026 2:01pm

[github-actions]

@claude review

[claude]

LGTM — small, well-scoped docs addition.

Extended reasoning...

Overview

The PR adds one new subsection ("Experiment instrumentation") to the v3→v4 self-hosting migration guide, plus a one-line clarification in the existing API table that POST /api/public/dataset-run-items is not part of the v4 write path. Only one file is touched, and the change is purely documentation prose — no code, config, or schema changes.

Security risks

None. This is static documentation content with no executable code, secrets, or user input handling.

Level of scrutiny

Low. This is a docs-only, additive change to a migration guide, following the same structure and heading conventions (H2/H3 with explicit [#anchor] tags) already used throughout the file.

Other factors

Verified both new links resolve: /docs/evaluation/experiments/experiments-via-sdk exists as a real page, and the OTEL anchor #experiments-ingesting-experiment-spans is explicitly defined via [#experiments-ingesting-experiment-spans] on the corresponding heading in content/integrations/native/opentelemetry/index.mdx. The file has exactly one H1 and only one H2/H3 level structure is used correctly, so it passes the H1 and heading-order checks. No md-override file exists for this route, so there is nothing to keep in sync. Content is consistent with the rest of the guide's tone and accurately reflects that the SDK-write path (dataset-run-items) is deprecated in favor of OTEL traces / the Experiment Runner SDK.