Document auto-configured Jetty metrics

https://github.com/spring-projects/spring-boot/issues/14591 added auto-configuration for binding Micrometer's `JettyServerThreadPoolMetrics` but neglected to add it to the documentation. It should be listed in the [section on supported metrics](https://docs.spring.io/spring-boot/docs/2.4.x/reference/html/production-ready-features.html#production-ready-metrics-meter).