Add an explicit note about having to enable cache metrics

See gh-23047

Author: snicoll

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/actuator/metrics.adoc

@@ -912,6 +912,9 @@ The following cache libraries are supported:
 * Any compliant JCache (JSR-107) implementation
 * Redis
 
+WARNING: Metrics should be enabled for the auto-configuration to pick them up.
+Refer to the documentation of the cache library you are using for more details.
+
 Metrics are tagged by the name of the cache and by the name of the javadoc:org.springframework.cache.CacheManager[], which is derived from the bean name.
 
 NOTE: Only caches that are configured on startup are bound to the registry.