Add details of the purpose of the metrics endpoint

See gh-44767

spring-boot-project/spring-boot-actuator-autoconfigure/src/docs/antora/modules/api/pages/rest/actuator/metrics.adoc

@@ -1,7 +1,7 @@
 [[metrics]]
 = Metrics (`metrics`)
 
-The `metrics` endpoint provides access to application metrics.
+The `metrics` endpoint provides access to application metrics to diagnose the metrics the application has recorded. This endpoint should not be "scraped" or used as a metrics backend in production. It's purpose is to show the currently registered metrics so users can see what metrics are available, what their current values are, and if triggering certain operations cause any change in certain values. If you want to diagnose your applications through the metrics they collect, you should use an external metrics backend. In this case, the `metrics` endpoint can still be useful.
 
 
 

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

@@ -66,7 +66,7 @@ Subject to xref:actuator/endpoints.adoc#actuator.endpoints.sanitization[sanitiza
   Requires one or more javadoc:{url-liquibase-javadoc}/liquibase.Liquibase[] beans.
 
 | `metrics`
-| Shows "`metrics`" information for the current application.
+| Shows "`metrics`" information for the current application to diagnose the metrics the application has recorded.
 
 | `mappings`
 | Displays a collated list of all javadoc:org.springframework.web.bind.annotation.RequestMapping[format=annotation] paths.