Polish "Improve docs on custom metrics"

Closes gh-12324

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

@@ -1749,19 +1749,12 @@ factories with a metric named `rabbitmq`.
 
 [[production-ready-metrics-custom]]
 === Registering custom metrics
-To register custom metrics, inject `MeterRegistry` into your component:
+To register custom metrics, inject `MeterRegistry` into your component, as shown in the
+following example:
 
 [source,java,indent=0]
 ----
-class Dictionary {
+include::{code-examples}/actuate/metrics/MetricsMeterRegistryInjectionExample.java[tag=component]
-	private List<String> words = new CopyOnWriteArrayList<>();
-
-	public MyComponent(MeterRegistry registry) {
-		registry.gaugeCollectionSize("dictionary.size", Tags.empty(), words);
-	}
-
-	...
-}
 ----
 
 If you find that you repeatedly instrument a suite of metrics across components or

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/actuate/metrics/MetricsMeterRegistryInjectionExample.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.actuate.metrics;
+
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.Tags;
+
+/**
+ * Example to show injection and use of a {@link MeterRegistry}.
+ *
+ * @author Andy Wilkinson
+ */
+public class MetricsMeterRegistryInjectionExample {
+
+	// tag::component[]
+	class Dictionary {
+
+		private final List<String> words = new CopyOnWriteArrayList<>();
+
+		Dictionary(MeterRegistry registry) {
+			registry.gaugeCollectionSize("dictionary.size", Tags.empty(), this.words);
+		}
+
+		// …
+
+	}
+	// end::component[]
+
+}