Align NettyProperties with Netty's defaults

Prior to this commit, `NettyProperties` would use `DISABLED` as the
default level for leak detection in Netty. This is not the default value
enforced by Netty, which is `SIMPLE`.

This commit changes the default configuration property value and ensures
that both stay in sync.

Fixes gh-27046

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/netty/NettyProperties.java

@@ -32,7 +32,7 @@ public class NettyProperties {
 	/**
 	 * Level of leak detection for reference-counted buffers.
 	 */
-	private LeakDetection leakDetection = LeakDetection.DISABLED;
+	private LeakDetection leakDetection = LeakDetection.SIMPLE;
 
 	public LeakDetection getLeakDetection() {
 		return this.leakDetection;

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/netty/NettyPropertiesTests.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2012-2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure.netty;
+
+import io.netty.util.ResourceLeakDetector;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link NettyProperties}
+ *
+ * @author Brian Clozel
+ */
+class NettyPropertiesTests {
+
+	@Test
+	void defaultValueShouldMatchNettys() {
+		NettyProperties properties = new NettyProperties();
+		assertThat(ResourceLeakDetector.Level.valueOf(properties.getLeakDetection().name()))
+				.isEqualTo(ResourceLeakDetector.getLevel());
+	}
+
+}