Property detect Health web extension with management context

Previously, the Health web extension was defined in the management
context and, as a result, it wasn't found when a separate port was
required. The side effect is that anything that the health web extension
does was not active anymore in that case.

This commit makes sure that the extension is always defined as part of
the main context where operations are discovered and merged.

Closes gh-11285

spring-boot-samples/spring-boot-sample-actuator/src/main/java/sample/actuator/ExampleHealthIndicator.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2012-2017 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
+ *
+ *      http://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 sample.actuator;
+
+import org.springframework.boot.actuate.health.Health;
+import org.springframework.boot.actuate.health.HealthIndicator;
+import org.springframework.stereotype.Component;
+
+@Component
+public class ExampleHealthIndicator implements HealthIndicator {
+
+	@Override
+	public Health health() {
+		return Health.up().withDetail("counter", 42).build();
+	}
+
+}