make metrics-core as provided dependency and refactor

spring-cloud-alibaba-examples/rocketmq-example/pom.xml

@@ -31,6 +31,12 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
+
+        <dependency>
+            <groupId>io.dropwizard.metrics</groupId>
+            <artifactId>metrics-core</artifactId>
+        </dependency>
+
     </dependencies>
 
     <build>