Document `spring.metrics.export.*` properties

See gh-3110

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -229,7 +229,7 @@ content into your application; rather pick only the properties that you need.
     spring.mustache.check-template-location=true
     spring.mustache.content-type=UTF-8
     spring.mustache.enabled=true # enable MVC view resolution
-    spring.mustache.prefix
+    spring.mustache.prefix=
     spring.mustache.suffix=.html
     spring.mustache.view-names= # whitelist of view names that can be resolved
 
@@ -652,6 +652,21 @@ content into your application; rather pick only the properties that you need.
 	shell.auth.simple.user.name=
 	shell.auth.simple.user.password=
 	shell.auth.spring.roles=
+	
+	# METRICS EXPORT ({sc-spring-boot-actuator}/metrics/export/MetricExportProperties.{sc-ext}[MetricExportProperties])
+	spring.metrics.export.enabled=true # Flag to disable all metric exports (assuming any MetricWriters are available).
+	spring.metrics.export.delay-millis=5000 # Delay in milliseconds between export ticks.
+	spring.metrics.export.send-latest=true # Flag to switch off any available optimizations based on not exporting unchanged metric values.
+	spring.metrics.export.includes= # List of patterns for metric names to include.
+	spring.metrics.export.excludes= # List of patterns for metric names to exclude. Applied after the includes.
+	spring.metrics.export.redis.prefix=spring.metrics # Prefix for redis repository if active.
+	spring.metrics.export.redis.key=keys.spring.metrics # Key for redis repository export (if active).
+	spring.metrics.export.triggers.<name>.names= # Names (or patterns) for bean names that this configuration applies to.
+	spring.metrics.export.triggers.<name>.enabled=true
+	spring.metrics.export.triggers.<name>.delay-millis=5000
+	spring.metrics.export.triggers.<name>.send-latest=true
+	spring.metrics.export.triggers.<name>.includes=
+	spring.metrics.export.triggers.<name>.excludes=
 
 	# SENDGRID ({sc-spring-boot-autoconfigure}/sendgrid/SendGridAutoConfiguration.{sc-ext}[SendGridAutoConfiguration])
 	spring.sendgrid.username= # SendGrid account username