Merge micrometer docs

Closes spring-projectsgh-11930

Author: snicoll

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