feat: Populate OpenFeature flag metadata from the evaluation reason

[kinyoklion]

Populates OpenFeature flag metadata with the LaunchDarkly specific parts of the evaluation result, so hooks (including the OpenTelemetry hook) can read them.

• Keys mirror the names of the evaluation reason fields: variationIndex, inExperiment, ruleIndex, ruleId, prerequisiteKey, bigSegmentsStatus.
• Each key is omitted rather than default-valued when it does not apply, so a consumer can distinguish "not in an experiment" from "no information".
• Matches openfeature-java-server#56; the naming is being defined in an sdk-specs spec.

No screenshots or staging preview apply — this is a server-side library change with no UI.

Implementation details

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

Describe the solution you've provided

EvaluationDetailExtensions gains a ToFlagMetadata extension on EvaluationReason, and both the success and error paths of ToResolutionDetails pass its result as flagMetadata:

private static ImmutableMetadata ToFlagMetadata(this EvaluationReason reason, int? variationIndex)
{
    var metadata = new Dictionary<string, object>();
    if (variationIndex.HasValue) metadata[VariationIndexKey] = variationIndex.Value;
    if (reason.InExperiment) metadata[InExperimentKey] = true;
    if (reason.RuleIndex.HasValue) metadata[RuleIndexKey] = reason.RuleIndex.Value;
    // ...
    return new ImmutableMetadata(metadata);
}

ruleIndex and ruleId are gated on the reason actually carrying them rather than on the reason kind, so no key is written from a sentinel value.

The README gains a "Flag Metadata" section documenting the keys.

Describe alternatives you've considered

A single nested reason structure would be closer to the LaunchDarkly evaluation reason, but ImmutableMetadata only holds scalar values, so the flat keys are the only shape available.

Testing

BUILDFRAMEWORKS=netstandard2.0 dotnet build src/LaunchDarkly.OpenFeature.ServerProvider -f netstandard2.0
BUILDFRAMEWORKS=net8.0 dotnet test test/LaunchDarkly.OpenFeature.ServerProvider.Tests -f net8.0

All tests pass, including 8 new cases covering each key, the omissions, and structure evaluations.

Additional context

net471 could not be exercised locally because Mono is not installed on the build machine; CI covers it.

@cursor review

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

---

Note

Overview
OpenFeature evaluation results now carry LaunchDarkly-specific metadata on ResolutionDetails.FlagMetadata, so hooks (e.g. OpenTelemetry) can read experiment, rule, and segment context without parsing the reason string alone.

EvaluationDetailExtensions maps EvaluationReason and variation index into flat keys—variationIndex, inExperiment, ruleIndex, ruleId, prerequisiteKey, and bigSegmentsStatus (as JSON-style status strings)—and passes that metadata on both successful and error conversions in ToResolutionDetails. Keys are omitted when they do not apply, not set to defaults.

The README adds a Flag Metadata section (key table and GetBooleanDetailsAsync example). Tests cover each key, omission behavior, and structure/Value evaluation paths.

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