Rename metrics, observation, and tracing modules to include micrometer in their names and packages

[wilkinsona]

• spring-boot-metrics -> spring-boot-micrometer-metrics
• spring-boot-observation -> spring-boot-micrometer-observation
• spring-boot-tracing -> spring-boot-micrometer-tracing

The packages should also change to be org.springframework.boot.micrometer.*.