Use reference stubs in Antora playbook for actuator-autoconfigure

Use reference stubs so that we can include a link to the supported
metrics backends.

Closes gh-45158

Author: philwebb

spring-boot-project/spring-boot-actuator-autoconfigure/build.gradle

@@ -212,6 +212,10 @@ def documentationTest = tasks.register("documentationTest", Test) {
 	}
 }
 
+tasks.named("generateAntoraPlaybook") {
+	antoraExtensions.xref.stubs = ["appendix:.*", "api:.*", "reference:.*"]
+}
+
 antoraContributions {
 	'actuator-rest-api' {
 		aggregateContent {

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

@@ -4,7 +4,7 @@
 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.
 Its 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.
+If you want to diagnose your applications through the metrics they collect, you should use an xref:reference:actuator/metrics.adoc[external metrics backend].
 In this case, the `metrics` endpoint can still be useful.