Extract metric export into a separate autoconfig class

This avoids a potential problems with ordering between Dropwizard and
normal repository configuration. A Dropwizard sample has been added to
verify the behaviour.

spring-boot-samples/spring-boot-sample-metrics-redis/src/test/java/sample/metrics/redis/SampleRedisExportApplicationTests.java

@@ -32,13 +32,12 @@ import org.springframework.test.context.web.WebAppConfiguration;
 @RunWith(SpringJUnit4ClassRunner.class)
 @SpringApplicationConfiguration(classes = SampleRedisExportApplication.class)
 @WebAppConfiguration
-@IntegrationTest("server.port=0")
+@IntegrationTest({ "server.port=0", "spring.jmx.enabled=true" })
 @DirtiesContext
 public class SampleRedisExportApplicationTests {
 
 	@Test
 	public void contextLoads() {
-
 	}
 
 }