fix: include experiment key on local-eval exposure events

[zhukaihan]

Summary

• Local evaluation exposure events from toExposureEvents carried experimentKey only inside the metadata blob — the top-level event property [Experiment] Experiment Key was never set, so custom reports keying off it saw no value for local-eval customers.
• Fix: when variant.Metadata["experimentKey"] is a non-empty string, set EventProperties["[Experiment] Experiment Key"]. Additive and backwards-compatible — metadata, [Experiment] Flag Key, [Experiment] Variant, user properties, and InsertID are unchanged.
• Mirrors amplitude/experiment-node-server#83.

Test plan

• Added with_experiment_key fixture to TestToExposureEvents. Asserts [Experiment] Experiment Key equals "exp-1" for that flag and is absent for the others.
• Updated event count expectation accordingly.
• go test ./pkg/experiment/local/ -run TestToExposureEvents -v — passes.

🤖 Generated with Claude Code

---

Note

Low Risk
Low risk: additive change that only sets an extra event property when variant.Metadata["experimentKey"] is present; existing event fields and filtering behavior are unchanged.

Overview
Local evaluation exposure tracking now lifts variant.Metadata["experimentKey"] into a top-level Amplitude event property ([Experiment] Experiment Key) in toExposureEvents, instead of leaving it only inside the metadata blob.

Tests were updated with a with_experiment_key fixture to assert the new property is set only when provided, and the expected event count was adjusted accordingly.

^{Reviewed by Cursor Bugbot for commit bcbd125. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

🎉 This PR is included in version 1.11.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀