Document Kafka metrics support

[hoplers]

Spring Boot 2.3 release notes states that it now supports native consumer & producer metrics (i guess this was implemented here - #20838), but documentation (latest,2.3.0,2.3.1,2.3.2) states that only kafka consumer metrics are supproted natively - for example https://docs.spring.io/spring-boot/docs/2.3.2.BUILD-SNAPSHOT/reference/htmlsingle/#production-ready-metrics-meter

[snicoll]

@hoplers good catch. I've also added a small section to provide additional details and link to Spring Kafka's documentation.

[hoplers]

Awesome, thank you!