fix: include experiment key on local-eval exposure events (#75)

Local evaluation exposure events emit `metadata.experimentKey` inside the
metadata blob but never lift it onto a top-level event property. Custom
reports keying off `[Experiment] Experiment Key` therefore see no value
for customers using local evaluation.

When `variant.metadata['experimentKey']` is present, set
`event_properties['[Experiment] Experiment Key']`. Mirrors the equivalent
fix in the Node SDK (amplitude/experiment-node-server#83).

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: zhukaihan

src/amplitude_experiment/exposure/exposure_service.py

@@ -41,6 +41,9 @@ def to_exposure_events(exposure: Exposure, ttl_millis: int) -> List[BaseEvent]:
             event_properties['[Experiment] Variant'] = variant.key
         elif variant.value:
             event_properties['[Experiment] Variant'] = variant.value
+        experiment_key = variant.metadata.get('experimentKey') if variant.metadata is not None else None
+        if experiment_key:
+            event_properties['[Experiment] Experiment Key'] = experiment_key
         if variant.metadata:
             event_properties['metadata'] = variant.metadata
 

tests/local/exposure/exposure_service_test.py

@@ -50,6 +50,13 @@ def test_to_exposure_events(self):
         })
         empty_metadata = Variant(key='on', value='on')
         empty_variant = Variant()
+        with_experiment_key = Variant(key='treatment', value='treatment', metadata={
+            'segmentName': 'All Other Users',
+            'flagType': 'experiment',
+            'flagVersion': 10,
+            'default': False,
+            'experimentKey': 'exp-1',
+        })
         results = {
             'basic': basic,
             'different_value': different_value,
@@ -59,12 +66,13 @@ def test_to_exposure_events(self):
             'partial_metadata': partial_metadata,
             'empty_metadata': empty_metadata,
             'empty_variant': empty_variant,
+            'with_experiment_key': with_experiment_key,
         }
         exposure = Exposure(user, results)
         events = to_exposure_events(exposure, DAY_MILLIS)
         # Should exclude default (default=True) only
-        # basic, different_value, mutex, holdout, partial_metadata, empty_metadata, empty_variant = 7 events
-        self.assertEqual(7, len(events))
+        # basic, different_value, mutex, holdout, partial_metadata, empty_metadata, empty_variant, with_experiment_key = 8 events
+        self.assertEqual(8, len(events))
 
         # Verify empty_variant is included
         empty_variant_events = [e for e in events if e.event_properties.get('[Experiment] Flag Key') == 'empty_variant']
@@ -95,6 +103,12 @@ def test_to_exposure_events(self):
             # If variant has no key or value (empty_variant), variant property may not be set
             if variant.metadata:
                 self.assertEqual(variant.metadata, event.event_properties.get('metadata'))
+
+            # Validate experiment key is lifted to top-level event property when present
+            if flag_key == 'with_experiment_key':
+                self.assertEqual('exp-1', event.event_properties.get('[Experiment] Experiment Key'))
+            else:
+                self.assertNotIn('[Experiment] Experiment Key', event.event_properties)
 
             # Validate user properties
             user_properties = event.user_properties