Flag main MbeanExporter `@Primary`

When the actuator is enabled, Spring Boot provides two `MBeanExporter`
bean definitions: a general purpose one and a dedicated one for Actuator
endpoints.

This commit flag the general purpose one `@Primary` so that component
can safely inject it by type if necessary. In particular, this fix the
doc of the `JmxMetricWriter`.

Closes gh-4007

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jmx/JmxAutoConfiguration.java

@@ -32,6 +32,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.EnableMBeanExport;
 import org.springframework.context.annotation.MBeanExportConfiguration.SpecificPlatform;
+import org.springframework.context.annotation.Primary;
 import org.springframework.core.env.Environment;
 import org.springframework.jmx.export.MBeanExporter;
 import org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource;
@@ -69,6 +70,7 @@ public class JmxAutoConfiguration implements EnvironmentAware, BeanFactoryAware
 	}
 
 	@Bean
+	@Primary
 	@ConditionalOnMissingBean(value = MBeanExporter.class, search = SearchStrategy.CURRENT)
 	public AnnotationMBeanExporter mbeanExporter(ObjectNamingStrategy namingStrategy) {
 		AnnotationMBeanExporter exporter = new AnnotationMBeanExporter();