fix: include usage_metadata events in live postprocessing

PiperOrigin-RevId: 870945434

Author: tilgalas

core/src/main/java/com/google/adk/flows/llmflows/BaseLlmFlow.java

@@ -611,7 +611,8 @@ private Flowable<Event> buildPostprocessingEvents(
     if (updatedResponse.content().isEmpty()
         && updatedResponse.errorCode().isEmpty()
         && !updatedResponse.interrupted().orElse(false)
-        && !updatedResponse.turnComplete().orElse(false)) {
+        && !updatedResponse.turnComplete().orElse(false)
+        && updatedResponse.usageMetadata().isEmpty()) {
       return processorEvents;
     }
 

core/src/test/java/com/google/adk/flows/llmflows/BaseLlmFlowTest.java

@@ -575,4 +575,35 @@ public Single<Map<String, Object>> runAsync(Map<String, Object> args, ToolContex
       return Single.just(response);
     }
   }
+
+  @Test
+  public void postprocess_noResponseProcessors_onlyUsageMetadata_returnsEvent() {
+    GenerateContentResponseUsageMetadata usageMetadata =
+        GenerateContentResponseUsageMetadata.builder()
+            .promptTokenCount(10)
+            .candidatesTokenCount(20)
+            .build();
+    LlmResponse llmResponse = LlmResponse.builder().usageMetadata(usageMetadata).build();
+    InvocationContext invocationContext =
+        createInvocationContext(createTestAgent(createTestLlm(llmResponse)));
+    BaseLlmFlow baseLlmFlow = createBaseLlmFlowWithoutProcessors();
+    Event baseEvent =
+        Event.builder()
+            .invocationId(invocationContext.invocationId())
+            .author(invocationContext.agent().name())
+            .build();
+
+    List<Event> events =
+        baseLlmFlow
+            .postprocess(invocationContext, baseEvent, LlmRequest.builder().build(), llmResponse)
+            .toList()
+            .blockingGet();
+
+    assertThat(events).hasSize(1);
+    Event event = getOnlyElement(events);
+    assertThat(event.content()).isEmpty();
+    assertThat(event.usageMetadata()).hasValue(usageMetadata);
+    assertThat(event.author()).isEqualTo(invocationContext.agent().name());
+    assertThat(event.invocationId()).isEqualTo(invocationContext.invocationId());
+  }
 }