Add auto-configuration for Micrometer 2.0.0 Observation API

- Adds a ObservationRegistry bean
- Add support for ObservationRegistryCustomizers
- Enables timer creation for observations if micrometer-core is on
  the classpath
- Registers ObservationPredicate, GlobalTagsProvider and
  ObservationHandler on the MeterRegistry
- Applies grouping to the ObservationHandlers: MeterObservationHandler
  are added to a FirstMatchingCompositeObservationHandler
- If micrometer-tracing is on the classpath, the
  TracingObservationHandler are added to a
  FirstMatchingCompositeObservationHandler

See spring-projectsgh-29666

spring-boot-project/spring-boot-actuator-autoconfigure/build.gradle

@@ -47,7 +47,9 @@ dependencies {
 	optional("com.zaxxer:HikariCP")
 	optional("io.dropwizard.metrics:metrics-jmx")
 	optional("io.lettuce:lettuce-core")
+	optional("io.micrometer:micrometer-observation")
 	optional("io.micrometer:micrometer-core")
+	optional("io.micrometer:micrometer-tracing-api")
 	optional("io.micrometer:micrometer-binders")
 	optional("io.micrometer:micrometer-registry-appoptics")
 	optional("io.micrometer:micrometer-registry-atlas") {

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/HandlersObservationRegistryCustomizer.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2012-2022 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.actuate.autoconfigure.observation;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.TreeMap;
+
+import io.micrometer.observation.ObservationHandler;
+import io.micrometer.observation.ObservationHandler.FirstMatchingCompositeObservationHandler;
+import io.micrometer.observation.ObservationRegistry;
+
+/**
+ * Installs {@link ObservationHandler} into an {@link ObservationRegistry}.
+ *
+ * Uses an {@link ObservationHandlerGrouping} strategy to group the handlers. All handlers
+ * which belong to a group are added with their group members into a
+ * {@link FirstMatchingCompositeObservationHandler}. All handlers without a group are
+ * added directly to the {@link ObservationRegistry}. The group numbers are ordered
+ * ascending, meaning lower numbers are added first to the {@link ObservationRegistry}.
+ *
+ * @author Moritz Halbritter
+ */
+class HandlersObservationRegistryCustomizer implements ObservationRegistryCustomizer<ObservationRegistry> {
+
+	private final List<ObservationHandler<?>> observationHandlers;
+
+	private final ObservationHandlerGrouping observationHandlerGrouping;
+
+	HandlersObservationRegistryCustomizer(List<ObservationHandler<?>> observationHandlers,
+			ObservationHandlerGrouping observationHandlerGrouping) {
+		this.observationHandlers = observationHandlers;
+		this.observationHandlerGrouping = observationHandlerGrouping;
+	}
+
+	@Override
+	public void customize(ObservationRegistry registry) {
+		Map<Integer, List<ObservationHandler<?>>> groupedHandlers = new TreeMap<>();
+		for (ObservationHandler<?> observationHandler : this.observationHandlers) {
+			Integer group = this.observationHandlerGrouping.getGroup(observationHandler);
+			if (group == null) {
+				registry.observationConfig().observationHandler(observationHandler);
+			}
+			else {
+				groupedHandlers.computeIfAbsent(group, (ignored) -> new ArrayList<>()).add(observationHandler);
+			}
+		}
+		for (List<ObservationHandler<?>> handlers : groupedHandlers.values()) {
+			registry.observationConfig()
+					.observationHandler(new FirstMatchingCompositeObservationHandler(castToRawType(handlers)));
+		}
+	}
+
+	@SuppressWarnings({ "unchecked", "rawtypes" })
+	private List<ObservationHandler> castToRawType(List<ObservationHandler<?>> handlers) {
+		// See https://github.com/micrometer-metrics/micrometer/issues/3064
+		return (List) handlers;
+	}
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/NoTracingObservationHandlerGrouping.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2012-2022 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.actuate.autoconfigure.observation;
+
+import io.micrometer.core.instrument.observation.MeterObservationHandler;
+import io.micrometer.observation.ObservationHandler;
+
+/**
+ * {@link ObservationHandler} grouping used by {@link ObservationAutoConfiguration} if
+ * micrometer-tracing is not on the classpath.
+ *
+ * @author Moritz Halbritter
+ */
+class NoTracingObservationHandlerGrouping implements ObservationHandlerGrouping {
+
+	private static final int METER_OBSERVATION_HANDLER_GROUP = 1;
+
+	@Override
+	public Integer getGroup(ObservationHandler<?> observationHandler) {
+		if (observationHandler instanceof MeterObservationHandler) {
+			return METER_OBSERVATION_HANDLER_GROUP;
+		}
+		return null;
+	}
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/ObservationAutoConfiguration.java

@@ -0,0 +1,105 @@
+/*
+ * Copyright 2012-2022 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.actuate.autoconfigure.observation;
+
+import java.util.List;
+
+import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.observation.Observation.GlobalTagsProvider;
+import io.micrometer.observation.ObservationHandler;
+import io.micrometer.observation.ObservationPredicate;
+import io.micrometer.observation.ObservationRegistry;
+import io.micrometer.tracing.handler.TracingObservationHandler;
+
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.actuate.autoconfigure.metrics.CompositeMeterRegistryAutoConfiguration;
+import org.springframework.boot.autoconfigure.AutoConfiguration;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * {@link EnableAutoConfiguration Auto-configuration} for the Micrometer Observation API.
+ *
+ * @author Moritz Halbritter
+ * @since 3.0.0
+ */
+@AutoConfiguration(after = CompositeMeterRegistryAutoConfiguration.class)
+@ConditionalOnClass(ObservationRegistry.class)
+public class ObservationAutoConfiguration {
+
+	@Bean
+	static ObservationRegistryPostProcessor observationRegistryPostProcessor(
+			ObjectProvider<ObservationRegistryCustomizer<?>> observationRegistryCustomizers,
+			ObjectProvider<ObservationPredicate> observationPredicates,
+			ObjectProvider<GlobalTagsProvider<?>> tagProviders) {
+		return new ObservationRegistryPostProcessor(observationRegistryCustomizers, observationPredicates,
+				tagProviders);
+	}
+
+	@Bean
+	@ConditionalOnMissingBean
+	ObservationRegistry observationRegistry() {
+		return ObservationRegistry.create();
+	}
+
+	@Bean
+	@ConditionalOnMissingBean
+	HandlersObservationRegistryCustomizer handlersObservationRegistryCustomizer(
+			List<ObservationHandler<?>> observationHandlers, ObservationHandlerGrouping observationHandlerGrouping) {
+		return new HandlersObservationRegistryCustomizer(observationHandlers, observationHandlerGrouping);
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	@ConditionalOnBean(MeterRegistry.class)
+	static class MetricsConfiguration {
+
+		@Bean
+		TimerObservationHandlerObservationRegistryCustomizer enableTimerObservationHandler(
+				MeterRegistry meterRegistry) {
+			return new TimerObservationHandlerObservationRegistryCustomizer(meterRegistry);
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	@ConditionalOnMissingClass("io.micrometer.tracing.handler.TracingObservationHandler")
+	static class NoTracingConfiguration {
+
+		@Bean
+		ObservationHandlerGrouping noTracingObservationHandlerGrouping() {
+			return new NoTracingObservationHandlerGrouping();
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	@ConditionalOnClass(TracingObservationHandler.class)
+	static class TracingConfiguration {
+
+		@Bean
+		ObservationHandlerGrouping tracingObservationHandlerGrouping() {
+			return new TracingObservationHandlerGrouping();
+		}
+
+	}
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/ObservationHandlerGrouping.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2012-2022 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.actuate.autoconfigure.observation;
+
+import io.micrometer.observation.ObservationHandler;
+
+/**
+ * Strategy to get the group for an {@link ObservationHandler}.
+ *
+ * @author Moritz Halbritter
+ * @since 3.0.0
+ */
+public interface ObservationHandlerGrouping {
+
+	/**
+	 * Returns the group number of the given {@code observationHandler}. If {@code null}
+	 * is returned, the {@link ObservationHandler} doesn't belong to any group.
+	 * @param observationHandler the observation handler to get the group for
+	 * @return group number or {@code null} if the handler doesn't belong to any group
+	 */
+	Integer getGroup(ObservationHandler<?> observationHandler);
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/ObservationRegistryConfigurer.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2012-2022 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.actuate.autoconfigure.observation;
+
+import java.util.List;
+
+import io.micrometer.observation.Observation.GlobalTagsProvider;
+import io.micrometer.observation.ObservationPredicate;
+import io.micrometer.observation.ObservationRegistry;
+
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.util.LambdaSafe;
+
+/**
+ * Configurer to apply {@link ObservationRegistryCustomizer customizers} to
+ * {@link ObservationRegistry observation registries}. Installs
+ * {@link ObservationPredicate observation predicates} and {@link GlobalTagsProvider
+ * global tag providers} into the {@link ObservationRegistry}.
+ *
+ * @author Moritz Halbritter
+ */
+class ObservationRegistryConfigurer {
+
+	private final ObjectProvider<ObservationRegistryCustomizer<?>> customizers;
+
+	private final ObjectProvider<ObservationPredicate> observationPredicates;
+
+	private final ObjectProvider<GlobalTagsProvider<?>> tagProviders;
+
+	ObservationRegistryConfigurer(ObjectProvider<ObservationRegistryCustomizer<?>> customizers,
+			ObjectProvider<ObservationPredicate> observationPredicates,
+			ObjectProvider<GlobalTagsProvider<?>> tagProviders) {
+		this.customizers = customizers;
+		this.observationPredicates = observationPredicates;
+		this.tagProviders = tagProviders;
+	}
+
+	void configure(ObservationRegistry registry) {
+		registerObservationPredicates(registry);
+		registerGlobalTagsProvider(registry);
+		customize(registry);
+	}
+
+	private void registerObservationPredicates(ObservationRegistry registry) {
+		this.observationPredicates.orderedStream().forEach(
+				(observationPredicate) -> registry.observationConfig().observationPredicate(observationPredicate));
+	}
+
+	private void registerGlobalTagsProvider(ObservationRegistry registry) {
+		this.tagProviders.orderedStream()
+				.forEach((tagProvider) -> registry.observationConfig().tagsProvider(tagProvider));
+	}
+
+	@SuppressWarnings("unchecked")
+	private void customize(ObservationRegistry registry) {
+		LambdaSafe.callbacks(ObservationRegistryCustomizer.class, asOrderedList(this.customizers), registry)
+				.withLogger(ObservationRegistryConfigurer.class).invoke((customizer) -> customizer.customize(registry));
+	}
+
+	private <T> List<T> asOrderedList(ObjectProvider<T> provider) {
+		return provider.orderedStream().toList();
+	}
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/ObservationRegistryCustomizer.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2012-2022 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.actuate.autoconfigure.observation;
+
+import io.micrometer.observation.ObservationRegistry;
+
+/**
+ * Callback interface that can be used to customize auto-configured
+ * {@link ObservationRegistry ObservationRegistries}.
+ *
+ * @param <T> the registry type to customize
+ * @author Moritz Halbritter
+ * @since 3.0.0
+ */
+@FunctionalInterface
+public interface ObservationRegistryCustomizer<T extends ObservationRegistry> {
+
+	/**
+	 * Customize the given {@code registry}.
+	 * @param registry the registry to customize
+	 */
+	void customize(T registry);
+
+}