Edit: Since I gave this answer a lot has changed. It was valid for 2.0.0.RC1. Please read the documentation https://docs.spring.io/spring-boot/docs/current/reference/html/production-ready-metrics.html
If above solution doesn't work for someone try this:
I had the same issue with Spring Boot 2.0.0.RC1, spring-boot-starter-web and of course spring-boot-starter-actuator.
My application.properties file read:
management.endpoints.web.expose=prometheus,metrics,info,health
In my pom file I had additionally:
<dependency>
<groupId>io.prometheus</groupId>
<artifactId>simpleclient</artifactId>
<version>0.2.0</version>
</dependency>
<dependency>
<groupId>io.micrometer</groupId>
<artifactId>micrometer-registry-prometheus</artifactId>
<version>0.12.0.RELEASE</version>
</dependency>
Prometheus metrics under /actuator/prometheus where only shown after I had switched to the newest version of micrometer-registry-prometheus:
<dependency>
<groupId>io.micrometer</groupId>
<artifactId>micrometer-registry-prometheus</artifactId>
<version>1.0.0-rc.9</version>
</dependency>