Add "Health as Gauge" how-to documentation

Add how-to documentation describing how health information can be
exported to a Micrometer Gauge.

Closes gh-18329

Author: philwebb

spring-boot-project/spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -2177,6 +2177,22 @@ The patterns to use can be customized using the `management.endpoint.env.keys-to
 
 
 
+[[howto-map-health-indicators-to-metrics]]
+=== Map Health Indicators to Micrometer Metrics
+Spring Boot health indicators return a `Status` type to indicate the overall system health.
+If you want to monitor or alert on levels of health for a particular application, you can export these statuses as metrics via Micrometer.
+By default, the status codes "`up`", "`down`", "`out of service`" and "`unknown`" are used by Spring Boot.
+To export these, you'll need to convert these states to some set of numbers so that they can be used with a Micrometer `Gauge`.
+
+The follow example shows one way to write such an exporter:
+
+[source,java,indent=0,subs="verbatim,quotes,attributes"]
+----
+include::{code-examples}/actuate/metrics/MetricsHealthMicrometerExportExample.java[tag=configuration]
+----
+
+
+
 [[howto-security]]
 == Security
 This section addresses questions about security when working with Spring Boot, including questions that arise from using Spring Security with Spring Boot.

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

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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.Map;
+
+import io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+
+import org.springframework.boot.actuate.health.CompositeHealthIndicator;
+import org.springframework.boot.actuate.health.HealthAggregator;
+import org.springframework.boot.actuate.health.HealthIndicator;
+import org.springframework.boot.actuate.health.Status;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Example to show how to export {@link HealthIndicator} beans to a {@link MeterRegistry}.
+ *
+ * @author Phillip Webb
+ */
+public class MetricsHealthMicrometerExportExample {
+
+	// tag::configuration[]
+	@Configuration
+	public class HealthMetricsConfiguration {
+
+		public HealthMetricsConfiguration(MeterRegistry registry, HealthAggregator aggregator,
+				Map<String, HealthIndicator> indicators) {
+			// This example presumes common tags (such as the app) are applied elsewhere
+			HealthIndicator health = new CompositeHealthIndicator(aggregator, indicators);
+			Gauge.builder("health", health, this::getStatusCode).strongReference(true).register(registry);
+		}
+
+		private int getStatusCode(HealthIndicator health) {
+			Status status = health.health().getStatus();
+			if (Status.UP.equals(status)) {
+				return 3;
+			}
+			if (Status.OUT_OF_SERVICE.equals(status)) {
+				return 2;
+			}
+			if (Status.DOWN.equals(status)) {
+				return 1;
+			}
+			return 0;
+		}
+
+	}
+	// end::configuration[]
+
+}

spring-boot-project/spring-boot-docs/src/test/java/org/springframework/boot/docs/actuate/metrics/MetricsHealthMicrometerExportExampleTests.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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 io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.actuate.autoconfigure.health.HealthIndicatorAutoConfiguration;
+import org.springframework.boot.actuate.autoconfigure.metrics.MetricsAutoConfiguration;
+import org.springframework.boot.actuate.health.Health;
+import org.springframework.boot.actuate.health.HealthIndicator;
+import org.springframework.boot.autoconfigure.ImportAutoConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Import;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link MetricsHealthMicrometerExportExample}.
+ *
+ * @author Phillip Webb
+ */
+@RunWith(SpringRunner.class)
+@SpringBootTest
+public class MetricsHealthMicrometerExportExampleTests {
+
+	@Autowired
+	private MeterRegistry registry;
+
+	@Test
+	public void registryExportsHealth() throws Exception {
+		Gauge gauge = this.registry.get("health").gauge();
+		assertThat(gauge.value()).isEqualTo(2);
+	}
+
+	@Configuration
+	@Import(MetricsHealthMicrometerExportExample.HealthMetricsConfiguration.class)
+	@ImportAutoConfiguration(classes = { HealthIndicatorAutoConfiguration.class, MetricsAutoConfiguration.class })
+	public static class Config {
+
+		@Bean
+		public MetricsHealthMicrometerExportExample example() {
+			return new MetricsHealthMicrometerExportExample();
+		}
+
+		@Bean
+		public SimpleMeterRegistry simpleMeterRegistry() {
+			return new SimpleMeterRegistry();
+		}
+
+		@Bean
+		public HealthIndicator outOfService() {
+			return () -> new Health.Builder().outOfService().build();
+		}
+
+	}
+
+}