Declare order for ServerHttpObservationFilter WebFilter

This commit declares an `@Order` for the `ServerHttpObservationFilter`
bean declaration in the Observation WebFlux auto-configuration.

This allows developers to consistently order other `WebFilter` instances
relatively to this one. Here, `@Order(Ordered.HIGHEST_PRECEDENCE + 1)`
has been chosen to align with the order of its MVC counterpart.

Fixes gh-33444

Author: bclozel

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/web/reactive/WebFluxObservationAutoConfiguration.java

@@ -42,6 +42,7 @@
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.Ordered;
 import org.springframework.core.annotation.Order;
 import org.springframework.http.server.reactive.observation.DefaultServerRequestObservationConvention;
 import org.springframework.http.server.reactive.observation.ServerRequestObservationConvention;
@@ -77,6 +78,7 @@ public WebFluxObservationAutoConfiguration(MetricsProperties metricsProperties,
 
 	@Bean
 	@ConditionalOnMissingBean
+	@Order(Ordered.HIGHEST_PRECEDENCE + 1)
 	public ServerHttpObservationFilter webfluxObservationFilter(ObservationRegistry registry,
 			ObjectProvider<ServerRequestObservationConvention> customConvention,
 			ObjectProvider<WebFluxTagsProvider> tagConfigurer,

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/observation/web/reactive/WebFluxObservationAutoConfigurationTests.java

@@ -16,11 +16,14 @@
 
 package org.springframework.boot.actuate.autoconfigure.observation.web.reactive;
 
+import java.util.List;
+
 import io.micrometer.core.instrument.MeterRegistry;
 import io.micrometer.core.instrument.Tag;
 import io.micrometer.core.instrument.Tags;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
+import reactor.core.publisher.Mono;
 
 import org.springframework.boot.actuate.autoconfigure.metrics.MetricsAutoConfiguration;
 import org.springframework.boot.actuate.autoconfigure.metrics.test.MetricsRun;
@@ -37,10 +40,14 @@
 import org.springframework.boot.test.system.OutputCaptureExtension;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
 import org.springframework.http.server.reactive.observation.DefaultServerRequestObservationConvention;
 import org.springframework.test.web.reactive.server.WebTestClient;
 import org.springframework.web.filter.reactive.ServerHttpObservationFilter;
 import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebFilter;
+import org.springframework.web.server.WebFilterChain;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -64,6 +71,16 @@ void shouldProvideWebFluxObservationFilter() {
 		this.contextRunner.run((context) -> assertThat(context).hasSingleBean(ServerHttpObservationFilter.class));
 	}
 
+	@Test
+	void shouldProvideWebFluxObservationFilterOrdered() {
+		this.contextRunner.withBean(FirstWebFilter.class).withBean(ThirdWebFilter.class).run((context) -> {
+			List<WebFilter> webFilters = context.getBeanProvider(WebFilter.class).orderedStream().toList();
+			assertThat(webFilters.get(0)).isInstanceOf(FirstWebFilter.class);
+			assertThat(webFilters.get(1)).isInstanceOf(ServerHttpObservationFilter.class);
+			assertThat(webFilters.get(2)).isInstanceOf(ThirdWebFilter.class);
+		});
+	}
+
 	@Test
 	void shouldUseConventionAdapterWhenCustomTagsProvider() {
 		this.contextRunner.withUserConfiguration(CustomTagsProviderConfiguration.class).run((context) -> {
@@ -207,4 +224,24 @@ static class CustomConvention extends DefaultServerRequestObservationConvention
 
 	}
 
+	@Order(Ordered.HIGHEST_PRECEDENCE)
+	static class FirstWebFilter implements WebFilter {
+
+		@Override
+		public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
+			return chain.filter(exchange);
+		}
+
+	}
+
+	@Order(Ordered.HIGHEST_PRECEDENCE + 2)
+	static class ThirdWebFilter implements WebFilter {
+
+		@Override
+		public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
+			return chain.filter(exchange);
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-docs/src/docs/asciidoc/web/reactive.adoc

@@ -207,6 +207,9 @@ When it does so, the orders shown in the following table will be used:
 |===
 | Web Filter | Order
 
+| `ServerHttpObservationFilter` (Micrometer Observability)
+| `Ordered.HIGHEST_PRECEDENCE + 1`
+
 | `WebFilterChainProxy` (Spring Security)
 | `-100`