Fix bug where metrics fail if actuator is not added as a project dependency

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -886,15 +886,17 @@ spring:
 
 === Gateway Metrics Filter
 
-The Gateway Metrics Filter runs as long as the property `spring.cloud.gateway.metrics.enabled` is not set to `false`. This filter adds a timer metric named "gateway.requests" with the following tags:
+To enable Gateway Metrics add spring-boot-starter-actuator as a project dependency. Then, by default, the Gateway Metrics Filter runs as long as the property `spring.cloud.gateway.metrics.enabled` is not set to `false`. This filter adds a timer metric named "gateway.requests" with the following tags:
 
 * `routeId`: The route id 
 * `routeUri`: The URI that the API will be routed to
-* `outcome` |Outcome as classified by link:https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/http/HttpStatus.Series.html[HttpStatus.Series]
+* `outcome`: Outcome as classified by link:https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/http/HttpStatus.Series.html[HttpStatus.Series]
 * `status`: Http Status of the request returned to the client
 
 These metrics are then available to be scraped from ``/actuator/metrics/gateway.requests`` and can be easily integated with Prometheus to create a link:images/gateway-grafana-dashboard.jpeg[Grafana] link:gateway-grafana-dashboard.json[dashboard].
 
+NOTE: To enable the pometheus endpoint add micrometer-registry-prometheus as a project dependency.
+
 === Making An Exchange As Routed
 
 After the Gateway has routed a `ServerWebExchange` it will mark that exchange as "routed" by adding `gatewayAlreadyRouted`

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/config/GatewayAutoConfiguration.java

@@ -17,12 +17,7 @@
 
 package org.springframework.cloud.gateway.config;
 
-import java.io.IOException;
-import java.net.URL;
-import java.security.cert.CertificateException;
-import java.security.cert.CertificateFactory;
 import java.security.cert.X509Certificate;
-import java.util.ArrayList;
 import java.util.List;
 import java.util.function.Consumer;
 
@@ -41,6 +36,8 @@ import rx.RxReactiveStreams;
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.boot.actuate.autoconfigure.endpoint.condition.ConditionalOnEnabledEndpoint;
+import org.springframework.boot.actuate.autoconfigure.metrics.CompositeMeterRegistryAutoConfiguration;
+import org.springframework.boot.actuate.autoconfigure.metrics.MetricsAutoConfiguration;
 import org.springframework.boot.actuate.health.Health;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
 import org.springframework.boot.autoconfigure.AutoConfigureBefore;
@@ -51,7 +48,6 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.boot.context.properties.PropertyMapper;
-import org.springframework.boot.web.server.WebServerException;
 import org.springframework.cloud.gateway.actuate.GatewayControllerEndpoint;
 import org.springframework.cloud.gateway.filter.AdaptCachedBodyGlobalFilter;
 import org.springframework.cloud.gateway.filter.ForwardPathFilter;
@@ -128,7 +124,6 @@ import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.DependsOn;
 import org.springframework.context.annotation.Primary;
 import org.springframework.http.codec.ServerCodecConfigurer;
-import org.springframework.util.ResourceUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.validation.Validator;
 import org.springframework.web.reactive.DispatcherHandler;
@@ -147,7 +142,9 @@ import static org.springframework.cloud.gateway.config.HttpClientProperties.Pool
 @ConditionalOnProperty(name = "spring.cloud.gateway.enabled", matchIfMissing = true)
 @EnableConfigurationProperties
 @AutoConfigureBefore(HttpHandlerAutoConfiguration.class)
-@AutoConfigureAfter({GatewayLoadBalancerClientAutoConfiguration.class, GatewayClassPathWarningAutoConfiguration.class})
+@AutoConfigureAfter({ GatewayLoadBalancerClientAutoConfiguration.class,
+		GatewayClassPathWarningAutoConfiguration.class, MetricsAutoConfiguration.class,
+		CompositeMeterRegistryAutoConfiguration.class })
 @ConditionalOnClass(DispatcherHandler.class)
 public class GatewayAutoConfiguration {
 
@@ -341,15 +338,18 @@ public class GatewayAutoConfiguration {
 		return new XForwardedHeadersFilter();
 	}
 
-
 	// GlobalFilter beans
 	
-	@Bean
-	@ConditionalOnProperty(name = "spring.cloud.gateway.metrics.enabled", matchIfMissing = true)	
-	public GatewayMetricsFilter gatewayMetricFilter(MeterRegistry meterRegistry) {
-		return new GatewayMetricsFilter(meterRegistry);
+	@Configuration
+	@ConditionalOnBean(MeterRegistry.class)
+	protected static class MetricsConfig {
+		@Bean
+		@ConditionalOnProperty(name = "spring.cloud.gateway.metrics.enabled", matchIfMissing = true)
+		public GatewayMetricsFilter gatewayMetricFilter(MeterRegistry meterRegistry) {
+			return new GatewayMetricsFilter(meterRegistry);
+		}
 	}
-
+	
 	@Bean
 	public AdaptCachedBodyGlobalFilter adaptCachedBodyGlobalFilter() {
 		return new AdaptCachedBodyGlobalFilter();

spring-cloud-gateway-sample/pom.xml

@@ -24,6 +24,12 @@
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-actuator</artifactId>
+			<exclusions> <!-- exclude metrics to verify that gateway doesn't fall over when it is not available -->
+				<exclusion>
+					<groupId>io.micrometer</groupId>
+					<artifactId>micrometer-core</artifactId>
+				</exclusion>
+			</exclusions>
 		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>