Updated the feign builder tracing logic

without this change a default Feign.Builder doesn't have its feign.Client instrumented
with this change we're using reflection to instrument that Client

fixes gh-1870

spring-cloud-sleuth-autoconfigure/src/main/java/org/springframework/cloud/sleuth/autoconfig/instrument/web/client/feign/TraceFeignClientAutoConfiguration.java

@@ -35,6 +35,7 @@ import org.springframework.cloud.sleuth.instrument.web.client.feign.FeignContext
 import org.springframework.cloud.sleuth.instrument.web.client.feign.OkHttpFeignClientBeanPostProcessor;
 import org.springframework.cloud.sleuth.instrument.web.client.feign.SleuthFeignBuilder;
 import org.springframework.cloud.sleuth.instrument.web.client.feign.TraceFeignAspect;
+import org.springframework.cloud.sleuth.instrument.web.client.feign.TraceFeignBuilderBeanPostProcessor;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
@@ -68,6 +69,11 @@ public class TraceFeignClientAutoConfiguration {
 			return new FeignContextBeanPostProcessor(beanFactory);
 		}
 
+		@Bean
+		static TraceFeignBuilderBeanPostProcessor traceFeignBuilderBeanPostProcessor(BeanFactory beanFactory) {
+			return new TraceFeignBuilderBeanPostProcessor(beanFactory);
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)

spring-cloud-sleuth-instrumentation/src/main/java/org/springframework/cloud/sleuth/instrument/web/client/feign/TraceFeignBuilderBeanPostProcessor.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2013-2021 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.cloud.sleuth.instrument.web.client.feign;
+
+import java.lang.reflect.Field;
+
+import feign.Client;
+import feign.Feign;
+
+import org.springframework.beans.BeansException;
+import org.springframework.beans.factory.BeanFactory;
+import org.springframework.beans.factory.config.BeanPostProcessor;
+import org.springframework.util.ReflectionUtils;
+
+/**
+ * {@link BeanPostProcessor} that ensures that each {@link Feign.Builder} has
+ * a trace representation of a {@link Client}.
+ *
+ * @since 3.0.2
+ * @author Marcin Grzejszczak
+ */
+public class TraceFeignBuilderBeanPostProcessor implements BeanPostProcessor {
+
+	private final BeanFactory beanFactory;
+
+	public TraceFeignBuilderBeanPostProcessor(BeanFactory beanFactory) {
+		this.beanFactory = beanFactory;
+	}
+
+	@Override
+	public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
+		if (bean instanceof Feign.Builder) {
+			Field client = ReflectionUtils.findField(Feign.Builder.class, "client");
+			ReflectionUtils.makeAccessible(client);
+			Feign.Builder delegate = (Feign.Builder) bean;
+			Client clientInDelegate = (Client) ReflectionUtils.getField(client, delegate);
+			if (clientInDelegate instanceof LazyClient || clientInDelegate instanceof LazyTracingFeignClient) {
+				return bean;
+			}
+			delegate.client(new LazyClient(this.beanFactory, clientInDelegate));
+			return bean;
+		}
+		return bean;
+	}
+
+}

spring-cloud-sleuth-instrumentation/src/main/java/org/springframework/cloud/sleuth/instrument/web/client/feign/TraceFeignObjectWrapper.java

@@ -70,8 +70,11 @@ final class TraceFeignObjectWrapper {
 
 	private Object loadBalancerClientFactory;
 
+	private final TraceFeignBuilderBeanPostProcessor traceFeignBuilderBeanPostProcessor;
+
 	TraceFeignObjectWrapper(BeanFactory beanFactory) {
 		this.beanFactory = beanFactory;
+		this.traceFeignBuilderBeanPostProcessor = new TraceFeignBuilderBeanPostProcessor(beanFactory);
 	}
 
 	Object wrap(Object bean) {
@@ -87,7 +90,7 @@ final class TraceFeignObjectWrapper {
 			}
 			return new LazyTracingFeignClient(this.beanFactory, (Client) bean);
 		}
-		return bean;
+		return this.traceFeignBuilderBeanPostProcessor.postProcessAfterInitialization(bean, null);
 	}
 
 	private Object instrumentedFeignLoadBalancerClient(Object bean) {