fix: include experiment key on local-eval exposure events

[zhukaihan]

Summary

• Local evaluation exposure events from to_exposure_events 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 present, set event_properties['[Experiment] Experiment Key']. Additive and backwards-compatible — metadata, [Experiment] Flag Key, [Experiment] Variant, user properties, and insert_id are unchanged.
• Mirrors amplitude/experiment-node-server#83.

Test plan

• Added with_experiment_key fixture to ExposureServiceTestCase.test_to_exposure_events. Asserts [Experiment] Experiment Key equals 'exp-1' for that flag and is absent for the others.
• Updated event count expectation accordingly.
• python -m unittest tests.local.exposure.exposure_service_test — 2 tests pass.

🤖 Generated with Claude Code

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Note

Low Risk
Low risk additive change to analytics event payloads: it only copies metadata.experimentKey into a new top-level event property and updates unit coverage.

Overview
Local-eval exposure events generated by to_exposure_events now lift variant.metadata['experimentKey'] into a top-level event property, setting [Experiment] Experiment Key when present (while still keeping the full metadata blob).

Tests extend ExposureServiceTestCase.test_to_exposure_events with a with_experiment_key fixture, update the expected event count, and assert the new property is present only for that flag and absent otherwise.

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