Add configuration property to enable micrometer annotations

Prior to this commit, the Micrometer annotations support (`@Timed`,
`@Counted`...) was guarded by the presence of both Micrometer and
AspectJ on the classpath.

This signal is too weak, considering the startup performance impact and
the fact that the AspectJ dependency can be brought transitively in many
cases.

This commit adds a new `micrometer.observations.annotations.enabled`
property that is set to `false` by default to only process the
annotations support when this property is enabled.

Fixes gh-39128

spring-boot-project/spring-boot-docs/src/docs/asciidoc/actuator/metrics.adoc

@@ -1067,7 +1067,8 @@ Metrics for Jetty's `Connector` instances are bound by using Micrometer's `Jetty
 
 [[actuator.metrics.supported.timed-annotation]]
 ==== @Timed Annotation Support
-To use `@Timed` where it is not directly supported by Spring Boot, refer to the {micrometer-concepts-docs}#_the_timed_annotation[Micrometer documentation].
+To enable scanning of `@Timed` annotations, you will need to set the configprop:micrometer.observations.annotations.enabled[] property to `true`.
+Please refer to the {micrometer-concepts-docs}#_the_timed_annotation[Micrometer documentation].
 
 
 

spring-boot-project/spring-boot-docs/src/docs/asciidoc/actuator/observability.adoc

@@ -85,3 +85,11 @@ NOTE: Spring Boot does not provide auto-configuration for OpenTelemetry metrics
 OpenTelemetry tracing is only auto-configured when used together with <<actuator#actuator.micrometer-tracing, Micrometer Tracing>>.
 
 The next sections will provide more details about logging, metrics and traces.
+
+
+
+[[actuator.observability.annotations]]
+=== Micrometer Observation Annotations support
+
+To enable scanning of metrics and tracing annotations like `@Timed`, `@Counted`, `@MeterTag` and `@NewSpan` annotations, you will need to set the configprop:micrometer.observations.annotations.enabled[] property to `true`.
+This feature is supported Micrometer directly, please refer to the {micrometer-concepts-docs}#_the_timed_annotation[Micrometer] and {micrometer-tracing-docs}/api.html#_aspect_oriented_programming[Micrometer Tracing] reference docs.

spring-boot-project/spring-boot-docs/src/docs/asciidoc/attributes.adoc

@@ -109,6 +109,7 @@
 :micrometer-docs: https://micrometer.io/docs
 :micrometer-concepts-docs: {micrometer-docs}/concepts
 :micrometer-registry-docs: {micrometer-docs}/registry
+:micrometer-tracing-docs: https://docs.micrometer.io/tracing/reference
 :tomcat-docs: https://tomcat.apache.org/tomcat-{tomcat-version}-doc
 :graal-version: 22.3
 :graal-native-image-docs: https://www.graalvm.org/{graal-version}/reference-manual/native-image