fix: add null checks for ApiTracerFactory in ClientContext

gax-java/gax/src/main/java/com/google/api/gax/rpc/ClientContext.java

@@ -274,7 +274,13 @@ public static ClientContext create(StubSettings settings) throws IOException {
         ApiTracerContext.newBuilder()
             .setServerAddress(endpointContext.resolvedServerAddress())
             .build();
-    ApiTracerFactory apiTracerFactory = settings.getTracerFactory().withContext(apiTracerContext);
+    ApiTracerFactory apiTracerFactory = settings.getTracerFactory();
+    if (apiTracerFactory != null) {
+      apiTracerFactory = apiTracerFactory.withContext(apiTracerContext);
+    }
+    if (apiTracerFactory == null) {
+      apiTracerFactory = BaseApiTracerFactory.getInstance();
+    }
 
     return newBuilder()
         .setBackgroundResources(backgroundResources.build())

gax-java/gax/src/test/java/com/google/api/gax/rpc/ClientContextTest.java

@@ -53,6 +53,8 @@
 import com.google.api.gax.rpc.testing.FakeClientSettings;
 import com.google.api.gax.rpc.testing.FakeStubSettings;
 import com.google.api.gax.rpc.testing.FakeTransportChannel;
+import com.google.api.gax.tracing.ApiTracerFactory;
+import com.google.api.gax.tracing.BaseApiTracerFactory;
 import com.google.auth.ApiKeyCredentials;
 import com.google.auth.CredentialTypeForMetrics;
 import com.google.auth.Credentials;
@@ -1287,4 +1289,35 @@ void test_nullMtlsEndpointIsNotPassedToTransportChannel() throws IOException {
     // is not passed to the TransportChannelProvider
     ClientContext.create(clientSettings);
   }
+
+  @Test
+  void testCreate_withNullTracerFactory() throws IOException {
+    FakeStubSettings.Builder builder = FakeStubSettings.newBuilder();
+    builder.setTransportChannelProvider(getFakeTransportChannelProvider());
+    builder.setCredentialsProvider(
+        FixedCredentialsProvider.create(Mockito.mock(Credentials.class)));
+
+    FakeStubSettings settings = Mockito.spy(builder.build());
+    Mockito.doReturn(null).when(settings).getTracerFactory();
+
+    ClientContext context = ClientContext.create(settings);
+    assertThat(context.getTracerFactory()).isSameInstanceAs(BaseApiTracerFactory.getInstance());
+  }
+
+  @Test
+  void testCreate_withTracerFactoryReturningNullWithContext() throws IOException {
+    FakeStubSettings.Builder builder = FakeStubSettings.newBuilder();
+    builder.setTransportChannelProvider(getFakeTransportChannelProvider());
+    builder.setCredentialsProvider(
+        FixedCredentialsProvider.create(Mockito.mock(Credentials.class)));
+
+    ApiTracerFactory apiTracerFactory = Mockito.mock(ApiTracerFactory.class);
+    Mockito.doReturn(null).when(apiTracerFactory).withContext(Mockito.any());
+
+    FakeStubSettings settings = Mockito.spy(builder.build());
+    Mockito.doReturn(apiTracerFactory).when(settings).getTracerFactory();
+
+    ClientContext context = ClientContext.create(settings);
+    assertThat(context.getTracerFactory()).isSameInstanceAs(BaseApiTracerFactory.getInstance());
+  }
 }