address review comments

instrumentation/spring/spring-boot-autoconfigure-3/README.md

@@ -0,0 +1,7 @@
+# Why do we need a separate module for Spring Boot 3 auto-configuration?
+
+`RestClientInstrumentationAutoConfiguration` imports `RestClientCustomizer`,
+which is part of Spring Boot 3 (rather than Spring framework).
+
+If we were to include this in the `spring-boot-autoconfigure` module, we would have to
+bump the Spring Boot version to 3, which would break compatibility with Spring Boot 2.

instrumentation/spring/spring-boot-autoconfigure-3/src/main/java/io/opentelemetry/instrumentation/spring/autoconfigure/instrumentation/web/RestClientBeanPostProcessor.java

@@ -22,11 +22,10 @@ public RestClientBeanPostProcessor(ObjectProvider<OpenTelemetry> openTelemetryPr
 
   @Override
   public Object postProcessAfterInitialization(Object bean, String beanName) {
-    if (!(bean instanceof RestClient)) {
-      return bean;
+    if (bean instanceof RestClient restClient) {
+      return addRestClientInterceptorIfNotPresent(restClient, openTelemetryProvider.getObject());
     }
-    return addRestClientInterceptorIfNotPresent(
-        ((RestClient) bean), openTelemetryProvider.getObject());
+    return bean;
   }
 
   private static RestClient addRestClientInterceptorIfNotPresent(

instrumentation/spring/spring-boot-autoconfigure-3/src/main/java/io/opentelemetry/instrumentation/spring/autoconfigure/instrumentation/web/RestClientInstrumentationAutoConfiguration.java

@@ -8,7 +8,6 @@
 import io.opentelemetry.api.OpenTelemetry;
 import io.opentelemetry.instrumentation.spring.autoconfigure.internal.SdkEnabled;
 import io.opentelemetry.instrumentation.spring.web.v3_1.SpringWebTelemetry;
-import io.opentelemetry.testing.internal.armeria.client.RestClient;
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
@@ -19,6 +18,7 @@
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.web.client.RestClient;
 
 /**
  * Configures {@link RestClient} for tracing.
@@ -33,8 +33,6 @@
 @Configuration
 public class RestClientInstrumentationAutoConfiguration {
 
-  public RestClientInstrumentationAutoConfiguration() {}
-
   @Bean
   static RestClientBeanPostProcessor otelRestClientBeanPostProcessor(
       ObjectProvider<OpenTelemetry> openTelemetryProvider) {