Merge micrometer docs

Closes gh-11930

spring-boot-project/spring-boot-docs/src/main/asciidoc/index.adoc

@@ -68,6 +68,7 @@ Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch; Andy Wilkinson;
 :hibernate-documentation: https://docs.jboss.org/hibernate/orm/5.2/userguide/html_single/Hibernate_User_Guide.html
 :jetty-documentation: https://www.eclipse.org/jetty/documentation/9.4.x
 :micrometer-concepts-documentation: https://micrometer.io/docs/concepts
+:micrometer-registry-documentation: http://micrometer.io/docs/registry
 :tomcat-documentation: https://tomcat.apache.org/tomcat-8.5-doc
 :kotlin-documentation: https://kotlinlang.org/docs/reference/
 :junit5-documentation: https://junit.org/junit5/docs/current/user-guide

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/actuate/metrics/SampleBean.java

+/*
+ * Copyright 2012-2018 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 org.springframework.boot.docs.actuate.metrics;
+
+import io.micrometer.core.instrument.Counter;
+import io.micrometer.core.instrument.MeterRegistry;
+
+import org.springframework.stereotype.Component;
+
+/**
+ * Example to show manual usage of {@link MeterRegistry}.
+ *
+ * @author Stephane Nicoll
+ */
+// tag::example[]
+@Component
+public class SampleBean {
+
+	private final Counter counter;
+
+	public SampleBean(MeterRegistry registry) {
+		this.counter = registry.counter("received.messages");
+	}
+
+	public void handleMessage(String message) {
+		this.counter.increment();
+		// handle message implementation
+	}
+
+}
+// end::example[]