Add JMX note for Kafka consumer metrics

See gh-19348

spring-boot-project/spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -1644,7 +1644,7 @@ Spring Boot registers the following core metrics when applicable:
 ** Number of classes loaded/unloaded
 * CPU metrics
 * File descriptor metrics
-* Kafka consumer metrics
+* Kafka consumer metrics (`spring.jmx.enabled` must be set to `true`)
 * Log4j2 metrics: record the number of events logged to Log4j2 at each level
 * Logback metrics: record the number of events logged to Logback at each level
 * Uptime metrics: report a gauge for uptime and a fixed gauge representing the application's absolute start time