add device id to feature flag context by default

src/androidTest/java/com/mixpanel/android/mpmetrics/FeatureFlagManagerTest.java

@@ -74,6 +74,7 @@ public JSONObject getRequestBodyAsJson() throws JSONException {
     private static class MockFeatureFlagDelegate implements FeatureFlagDelegate {
         MPConfig configToReturn;
         String distinctIdToReturn = TEST_DISTINCT_ID;
+        String anonymousIdToReturn = "test_anonymous_id";
         String tokenToReturn = TEST_TOKEN;
         List<TrackCall> trackCalls = new ArrayList<>();
         CountDownLatch trackCalledLatch; // Optional: for tests waiting for track
@@ -101,6 +102,11 @@ public String getDistinctId() {
             return distinctIdToReturn;
         }
 
+        @Override
+        public String getAnonymousId() {
+            return anonymousIdToReturn;
+        }
+
         @Override
         public void track(String eventName, JSONObject properties) {
             MPLog.v("FeatureFlagManagerTest", "MockDelegate.track called: " + eventName);
@@ -453,6 +459,18 @@ public void testTracking_getVariantSync_calledOnce() throws InterruptedException
         assertEquals("$experiment_started", call.eventName);
         assertEquals("track_flag_sync", call.properties.getString("Experiment name"));
         assertEquals("v_track_sync", call.properties.getString("Variant name"));
+        assertEquals("feature_flag", call.properties.getString("$experiment_type"));
+
+        // Verify that timing properties are included
+        assertTrue("timeLastFetched should be present", call.properties.has("timeLastFetched"));
+        assertTrue("fetchLatencyMs should be present", call.properties.has("fetchLatencyMs"));
+
+        // Verify that timing values are reasonable
+        long timeLastFetched = call.properties.getLong("timeLastFetched");
+        long fetchLatencyMs = call.properties.getLong("fetchLatencyMs");
+        assertTrue("timeLastFetched should be positive", timeLastFetched > 0);
+        assertTrue("fetchLatencyMs should be non-negative", fetchLatencyMs >= 0);
+        assertTrue("fetchLatencyMs should be reasonable (< 5 seconds)", fetchLatencyMs < 5000);
     }
 
     @Test
@@ -940,6 +958,11 @@ public void testRequestBodyConstruction_performFetchRequest() throws Interrupted
         assertEquals("Context should contain correct distinct_id", 
                      "test_user_123", requestContext.getString("distinct_id"));
 
+        // Verify device_id is in the context
+        assertTrue("Context should contain device_id", requestContext.has("device_id"));
+        assertEquals("Context should contain correct device_id", 
+                     "test_anonymous_id", requestContext.getString("device_id"));
+
         // Verify the context contains the expected properties from FlagsConfig
         assertEquals("Context should contain $os", "Android", requestContext.getString("$os"));
         assertEquals("Context should contain $os_version", "13", requestContext.getString("$os_version"));
@@ -962,8 +985,9 @@ public void testRequestBodyConstruction_withNullContext() throws InterruptedExce
         // Setup with flags enabled but null context
         setupFlagsConfig(true, null);
 
-        // Set distinct ID
+        // Set distinct ID and anonymous ID
         mMockDelegate.distinctIdToReturn = "user_456";
+        mMockDelegate.anonymousIdToReturn = "device_789";
 
         // Create response
         Map<String, MixpanelFlagVariant> serverFlags = new HashMap<>();
@@ -987,9 +1011,13 @@ public void testRequestBodyConstruction_withNullContext() throws InterruptedExce
         // Verify distinct_id is in context
         assertEquals("Context should contain correct distinct_id", "user_456", requestContext.getString("distinct_id"));
 
-        // When FlagsConfig context is null, the context object should only contain distinct_id
-        assertEquals("Context should only contain distinct_id when FlagsConfig context is null", 
-                     1, requestContext.length());
+        // Verify device_id is in context
+        assertTrue("Context should contain device_id", requestContext.has("device_id"));
+        assertEquals("Context should contain correct device_id", "device_789", requestContext.getString("device_id"));
+
+        // When FlagsConfig context is null, the context object should only contain distinct_id and device_id
+        assertEquals("Context should only contain distinct_id and device_id when FlagsConfig context is null", 
+                     2, requestContext.length());
     }
 
     @Test
@@ -999,6 +1027,7 @@ public void testRequestBodyConstruction_withEmptyDistinctId() throws Interrupted
 
         // Set empty distinct ID
         mMockDelegate.distinctIdToReturn = "";
+        mMockDelegate.anonymousIdToReturn = "device_empty_test";
 
         // Create response
         Map<String, MixpanelFlagVariant> serverFlags = new HashMap<>();
@@ -1022,6 +1051,10 @@ public void testRequestBodyConstruction_withEmptyDistinctId() throws Interrupted
         // Verify distinct_id is included in context even when empty
         assertTrue("Context should contain distinct_id field", requestContext.has("distinct_id"));
         assertEquals("Context should contain empty distinct_id", "", requestContext.getString("distinct_id"));
+
+        // Verify device_id is included in context
+        assertTrue("Context should contain device_id field", requestContext.has("device_id"));
+        assertEquals("Context should contain correct device_id", "device_empty_test", requestContext.getString("device_id"));
     }
 
     @Test
@@ -1114,9 +1147,11 @@ public void testFlagsConfigContextUsage_emptyContext() throws InterruptedExcepti
         JSONObject requestBody = capturedRequest.getRequestBodyAsJson();
         JSONObject requestContext = requestBody.getJSONObject("context");
 
-        // Context should only contain distinct_id when initial context is empty
-        assertEquals("Context should only contain distinct_id", 1, requestContext.length());
+        // Context should only contain distinct_id and device_id when initial context is empty
+        assertEquals("Context should only contain distinct_id and device_id", 2, requestContext.length());
         assertEquals("distinct_id should be present", "test_user", requestContext.getString("distinct_id"));
+        assertTrue("device_id should be present", requestContext.has("device_id"));
+        assertEquals("device_id should be present", "test_anonymous_id", requestContext.getString("device_id"));
     }
 
     @Test

src/main/java/com/mixpanel/android/mpmetrics/FeatureFlagDelegate.java

@@ -9,6 +9,7 @@
 interface FeatureFlagDelegate {
     MPConfig getMPConfig();
     String getDistinctId();
+    String getAnonymousId();
     void track(String eventName, JSONObject properties);
     String getToken();
 }

src/main/java/com/mixpanel/android/mpmetrics/FeatureFlagManager.java

@@ -49,6 +49,9 @@ class FeatureFlagManager implements MixpanelAPI.Flags {
     private final Set<String> mTrackedFlags = new HashSet<>();
     private boolean mIsFetching = false;
     private List<FlagCompletionCallback<Boolean>> mFetchCompletionCallbacks = new ArrayList<>();
+    // Track last fetch time and latency for $experiment_started event
+    private volatile long mTimeLastFetched = 0L;
+    private volatile long mFetchLatencyMs = 0L;
     // ---
 
     // Message codes for Handler
@@ -430,6 +433,7 @@ private void _fetchFlagsIfNeeded(@Nullable FlagCompletionCallback<Boolean> compl
      * back to the mHandler thread via MSG_COMPLETE_FETCH.
      */
     private void _performFetchRequest() {
+        long fetchStart = System.currentTimeMillis();
         MPLog.v(LOGTAG, "Performing fetch request on thread: " + Thread.currentThread().getName());
         boolean success = false;
         JSONObject responseJson = null; // To hold parsed successful response
@@ -444,6 +448,7 @@ private void _performFetchRequest() {
 
         final MPConfig config = delegate.getMPConfig();
         final String distinctId = delegate.getDistinctId();
+        final String deviceId = delegate.getAnonymousId();
 
         if (distinctId == null) {
             MPLog.w(LOGTAG, "Distinct ID is null. Cannot fetch flags.");
@@ -456,6 +461,9 @@ private void _performFetchRequest() {
             // 1. Build Request Body JSON
             JSONObject contextJson = new JSONObject(mFlagsConfig.context.toString());
             contextJson.put("distinct_id", distinctId);
+            if (deviceId != null) {
+                contextJson.put("device_id", deviceId);
+            }
             JSONObject requestJson = new JSONObject();
             requestJson.put("context", contextJson);
             String requestJsonString = requestJson.toString();
@@ -527,6 +535,9 @@ private void _performFetchRequest() {
             MPLog.e(LOGTAG, errorMessage, e);
         }
 
+        long fetchEnd = System.currentTimeMillis();
+        mTimeLastFetched = fetchEnd;
+        mFetchLatencyMs = fetchEnd - fetchStart;
         // 6. Post result back to Handler thread
         postResultToHandler(success, responseJson, errorMessage);
     }
@@ -625,6 +636,8 @@ private void _performTrackingDelegateCall(String flagName, MixpanelFlagVariant v
             properties.put("Experiment name", flagName);
             properties.put("Variant name", variant.key); // Use the variant key
             properties.put("$experiment_type", "feature_flag");
+            properties.put("timeLastFetched", mTimeLastFetched);
+            properties.put("fetchLatencyMs", mFetchLatencyMs);
         } catch (JSONException e) {
             MPLog.e(LOGTAG, "Failed to create JSON properties for $experiment_started event", e);
             return; // Don't track if properties failed