Make sure metricsExecutor is shutdown cleanly

The default executor for publishing metrics was a single
thread (native JDK) ExecutorService with a non-daemon thread.
Changed it to a Spring-managed thread pool of size 1, which
is then shutdown with e ApplicationContext automatically.

Fixes gh-388

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/MetricRepositoryAutoConfiguration.java

@@ -18,7 +18,6 @@ package org.springframework.boot.actuate.autoconfigure;
 
 import java.util.List;
 import java.util.concurrent.Executor;
-import java.util.concurrent.Executors;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
@@ -44,6 +43,7 @@ import org.springframework.context.annotation.Primary;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.SubscribableChannel;
 import org.springframework.messaging.support.ExecutorSubscribableChannel;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
 
 import com.codahale.metrics.MetricRegistry;
 
@@ -121,9 +121,9 @@ public class MetricRepositoryAutoConfiguration {
 	@ConditionalOnClass(MessageChannel.class)
 	static class MetricsChannelConfiguration {
 
-		@Autowired(required = false)
+		@Autowired
 		@Qualifier("metricsExecutor")
-		private final Executor executor = Executors.newSingleThreadExecutor();
+		private Executor executor;
 
 		@Bean
 		@ConditionalOnMissingBean(name = "metricsChannel")
@@ -131,6 +131,13 @@ public class MetricRepositoryAutoConfiguration {
 			return new ExecutorSubscribableChannel(this.executor);
 		}
 
+		@Bean
+		@ConditionalOnMissingBean(name = "metricsExecutor")
+		protected Executor metricsExecutor() {
+			ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+			return executor;
+		}
+
 		@Bean
 		@Primary
 		@ConditionalOnMissingBean(name = "primaryMetricWriter")

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/MetricRepositoryAutoConfigurationTests.java

@@ -30,6 +30,8 @@ import org.springframework.context.annotation.AnnotationConfigApplicationContext
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.task.SyncTaskExecutor;
+import org.springframework.messaging.support.ExecutorSubscribableChannel;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
 
 import com.codahale.metrics.Gauge;
 import com.codahale.metrics.MetricRegistry;
@@ -38,6 +40,7 @@ import static org.hamcrest.Matchers.equalTo;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertThat;
+import static org.junit.Assert.assertTrue;
 import static org.mockito.Matchers.any;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
@@ -50,6 +53,17 @@ import static org.mockito.Mockito.verify;
  */
 public class MetricRepositoryAutoConfigurationTests {
 
+	@Test
+	public void defaultExecutor() throws Exception {
+		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(
+				MetricRepositoryAutoConfiguration.class);
+		ExecutorSubscribableChannel channel = context
+				.getBean(ExecutorSubscribableChannel.class);
+		ThreadPoolTaskExecutor executor = (ThreadPoolTaskExecutor) channel.getExecutor();
+		context.close();
+		assertTrue(executor.getThreadPoolExecutor().isShutdown());
+	}
+
 	@Test
 	public void createServices() throws Exception {
 		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(