Polish "Restore behavior of management.metrics.export.simple.enabled"

Closes gh-12106

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/simple/SimpleMetricsExportAutoConfiguration.java

@@ -47,7 +47,7 @@ import org.springframework.context.annotation.Configuration;
 @ConditionalOnBean(Clock.class)
 @EnableConfigurationProperties(SimpleProperties.class)
 @ConditionalOnMissingBean(MeterRegistry.class)
-@ConditionalOnProperty(prefix = "management.metrics.export.simple", name = "enabled", havingValue = "true", matchIfMissing = true)
+@ConditionalOnProperty(name = "management.metrics.export.simple.enabled", havingValue = "true", matchIfMissing = true)
 public class SimpleMetricsExportAutoConfiguration {
 
 	@Bean

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/simple/SimpleProperties.java

@@ -33,12 +33,6 @@ import org.springframework.boot.context.properties.ConfigurationProperties;
 @ConfigurationProperties(prefix = "management.metrics.export.simple")
 public class SimpleProperties {
 
-	/**
-	 * Enable in-memory metrics that aren't published anywhere (allows you to see
-	 * what metrics are collected in the metrics actuator endpoint).
-	 */
-	private boolean enabled;
-
 	/**
 	 * Step size (i.e. reporting frequency) to use.
 	 */
@@ -49,14 +43,6 @@ public class SimpleProperties {
 	 */
 	private CountingMode mode = CountingMode.CUMULATIVE;
 
-	public boolean getEnabled() {
-		return this.enabled;
-	}
-
-	public void setEnabled(boolean enabled) {
-		this.enabled = enabled;
-	}
-
 	public Duration getStep() {
 		return this.step;
 	}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -215,6 +215,12 @@
       "description": "Whether to enable uptime metrics.",
       "defaultValue": true
     },
+    {
+      "name": "management.metrics.export.simple.enabled",
+      "type": "java.lang.Boolean",
+      "description": "Whether, in the absence of any other exporter, exporting of metrics to an in-memory backend is enabled.",
+      "defaultValue": true
+    },
     {
       "name": "management.trace.http.enabled",
       "type": "java.lang.Boolean",

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

@@ -1384,7 +1384,7 @@ content into your application. Rather, pick only the properties that you need.
 	management.metrics.export.signalfx.uri= # Optional custom URI for the SignalFX API.
 	management.metrics.export.simple.mode=cumulative # Counting mode.
 	management.metrics.export.simple.step=10s # Step size (i.e. reporting frequency) to use.
-	management.metrics.export.statsd.enabled= # Whether exporting of metrics to this backend is enabled.
+	management.metrics.export.statsd.enabled=true # Whether, in the absence of any other exporter, exporting of metrics to an in-memory backend is enabled.
 	management.metrics.export.statsd.flavor=datadog # StatsD line protocol to use.
 	management.metrics.export.statsd.host=localhost # Host of the StatsD server to receive exported metrics.
 	management.metrics.export.statsd.max-packet-length=1400 # Total length of a single payload should be kept within your network's MTU.

spring-boot-project/spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -1272,6 +1272,22 @@ using:
 
 
 
+[[production-ready-metrics-export-simple]]
+==== Simple
+Micrometer ships with a simple, in-memory backend that is automatically used as a fallback
+if no other registry is configured. This allows you to see what metrics are collected in
+the <<production-ready-metrics-endpoint,metrics endpoint>>.
+
+The in-memory backend disables itself as soon as you're using any of the other available
+backend. You can also disable it explicitly:
+
+[source,properties,indent=0]
+----
+	management.metrics.export.simple.enabled=false
+----
+
+
+
 [[production-ready-metrics-export-newrelic]]
 ==== New Relic
 New Relic registry pushes metrics to New Relic periodically. To export metrics to