feat(llmobs): implement multi-tenant routing context support

[PROFeNoM]

What does this PR do?

Adds multi-tenant API key support for LLM Observability, enabling platforms like Dust.tt to route LLMObs telemetry to different Datadog organizations from a single process.

New API:

await llmobs.withRoutingContext({ ddApiKey: customerKey, ddSite: customerSite }, async () => {
  // All LLMObs spans created here will be routed to the customer's Datadog org
})

Flow:

User Code → withRoutingContext() → Span Created (routing tags captured) → 
Span Finished → SpanProcessor → Writer (routes to correct buffer/endpoint)

Key changes:

• Added withRoutingContext(options, fn) method to LLMObs SDK using AsyncLocalStorage
• Routing context is captured at span creation time and stored in span metadata
• Modified BaseLLMObsWriter to use a multi-buffer architecture (one buffer per apiKey:site combination)
• Each buffer flushes independently to its corresponding Datadog organization endpoint
• Works in agentless mode only (per design decision)

Motivation

Multi-tenant AI platforms (like Dust.tt) need to send LLM Observability data to their customers' Datadog organizations rather than a single shared organization. This enables:

• Customer-specific trace visibility in their own Datadog account
• Data isolation between tenants
• Compliance with data residency requirements (different DD sites per customer)

Plugin Checklist

• Unit tests.
• Integration tests.
• Benchmarks.
• TypeScript definitions.
• TypeScript tests.
• API documentation.
• CI jobs/workflows.

Additional Notes

[codecov]

Codecov Report

❌ Patch coverage is 65.33333% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.81%. Comparing base (a5fcf17) to head (059c462).

Files with missing lines	Patch %	Lines
packages/dd-trace/src/llmobs/writers/base.js	70.83%	14 Missing ⚠️
packages/dd-trace/src/llmobs/routing-context.js	44.44%	5 Missing ⚠️
packages/dd-trace/src/llmobs/tagger.js	42.85%	4 Missing ⚠️
packages/dd-trace/src/llmobs/sdk.js	33.33%	2 Missing ⚠️
packages/dd-trace/src/llmobs/noop.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7158      +/-   ##
==========================================
- Coverage   84.48%   83.81%   -0.68%     
==========================================
  Files         523      522       -1     
  Lines       22448    22413      -35     
==========================================
- Hits        18966    18786     -180     
- Misses       3482     3627     +145     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[PROFeNoM]

@codex review

[github-actions]

Overall package size

Self size: 4.4 MB
Deduped: 5.22 MB
No deduping: 5.22 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | import-in-the-middle | 2.0.0 | 68.46 kB | 797.03 kB | | dc-polyfill | 0.1.10 | 26.73 kB | 26.73 kB |

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}

[chatgpt-codex-connector]

To use Codex here, create a Codex account and connect to github.

[datadog-datadog-prod-us1]

⚠️ Tests

✨ Fix all issues with Cursor

⚠️ Warnings

❄️ 1 New flaky test detected

cypress@14.5.4 esm test management quarantine fails if quarantine is not enabled from integration-tests/cypress/cypress.spec.js (Datadog) (Fix with Cursor)

Cannot read properties of undefined (reading 'content')

ℹ️ Info

🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 059c462 | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-12-23 10:23:50

Comparing candidate commit 059c462 in PR branch alex/MLOB-4999_multi-tenant-routing-context-support with baseline commit a5fcf17 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 289 metrics, 31 unstable metrics.