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

buildSrc/src/main/java/org/springframework/boot/build/context/properties/DocumentConfigurationProperties.java

@@ -215,6 +215,7 @@ public class DocumentConfigurationProperties extends DefaultTask {
 
 	private void actuatorPrefixes(Config prefix) {
 		prefix.accept("management");
+		prefix.accept("micrometer");
 	}
 
 	private void dockerComposePrefixes(Config prefix) {