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

Closes gh-12106

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