feat: Populate OpenFeature flag metadata from the evaluation reason

[kinyoklion]

Populates OpenFeature flag metadata from the LaunchDarkly evaluation reason so consumers (and the OpenFeature/OTel telemetry hooks) can read experiment, rule, prerequisite, and Big Segments information.

• Adds flat, camelCase keys variationIndex, inExperiment, ruleIndex, ruleId, prerequisiteKey, bigSegmentsStatus; each is omitted when it does not apply.
• Applies to the shared server translator, so it covers the Node provider (and any future server provider built on it).
• Matches the behavior now specified in sdk-specs#253 and implemented in the Java, .NET, Python, and Ruby providers.
• Documents the keys in the Node provider README.

@cursor review

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Implementation details

translateResult now builds a FlagMetadata object from LDEvaluationDetail:

• variationIndex is included only when the SDK returned a variation, so default/error evaluations omit it.
• inExperiment is emitted only when true, rather than emitting false for every non-experiment evaluation.
• ruleIndex/ruleId, prerequisiteKey, and bigSegmentsStatus are copied through when the reason provides them.

Key names are named constants so they cannot drift from the spec.

Backend-only change; no UI, so no screenshots or staging preview apply.

Testing: yarn workspace @launchdarkly/openfeature-js-server-common test and ... lint, plus yarn workspace @launchdarkly/openfeature-node-server test. The existing BaseOpenFeatureProvider result assertions were updated for the added flagMetadata field.

Describe alternatives you've considered

Emitting the full reason object under a single nested key was rejected — OpenFeature flag metadata values must be scalars, and flat keys are what the spec draft standardizes across providers.

Link to Devin session: https://app.devin.ai/sessions/0c452d209ec54b068ba120b4c92b8f6c
Requested by: @kinyoklion

---

Note

Overview
OpenFeature resolution results from the shared server provider now include LaunchDarkly-specific flagMetadata on every successful translation path, built from LDEvaluationDetail in translateResult.

translateResult maps optional scalar keys when they apply: variationIndex (skipped for null/missing variation), inExperiment (only when true), plus ruleIndex/ruleId, prerequisiteKey, and bigSegmentsStatus from the evaluation reason. The LDFlagMetadata type is exported from @launchdarkly/openfeature-js-server-common, so Node and future server providers inherit the behavior automatically.

The Node OpenFeature README adds a Flag Metadata section documenting the keys and a short usage example. Tests cover metadata shaping and update provider integration expectations to include flagMetadata.

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[devin-ai-integration]

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26360 bytes
Compressed size limit: 29000
Uncompressed size: 129188 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179887 bytes
Compressed size limit: 200000
Uncompressed size: 833430 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32444 bytes
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Uncompressed size: 116232 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 41502 bytes
Compressed size limit: 44000
Uncompressed size: 224541 bytes

[kinyoklion]

Could we not use typing here? Instead of constants?

[devin-ai-integration]

Yes — replaced the constants with an exported LDFlagMetadata type (FlagMetadata intersected with the documented LaunchDarkly keys), so the keys and their value types are checked at the assignment sites and the metadata shape is now documented in the API reference. Package lint, build, and tests pass.