Fix RestClientBeanPostProcessor to avoid creating new bean when interceptor already present

The RestClientBeanPostProcessor was always creating a new RestClient bean
via mutate().build(), even when the OpenTelemetry interceptor was already
present. This resulted in unnecessary bean creation.

Fixes #15545

Signed-off-by: Lenin Jaganathan<lenin.jaganathan@gmail.com>

Author: Lenin Jaganathan

instrumentation/spring/spring-boot-autoconfigure/src/main/javaSpring3/io/opentelemetry/instrumentation/spring/autoconfigure/internal/instrumentation/web/RestClientBeanPostProcessor.java

@@ -10,6 +10,7 @@
 import io.opentelemetry.instrumentation.spring.autoconfigure.internal.properties.InstrumentationConfigUtil;
 import io.opentelemetry.instrumentation.spring.web.v3_1.SpringWebTelemetry;
 import io.opentelemetry.instrumentation.spring.web.v3_1.internal.WebTelemetryUtil;
+import java.util.concurrent.atomic.AtomicBoolean;
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.config.BeanPostProcessor;
 import org.springframework.http.client.ClientHttpRequestInterceptor;
@@ -40,18 +41,26 @@ private static RestClient addRestClientInterceptorIfNotPresent(
       RestClient restClient, OpenTelemetry openTelemetry, InstrumentationConfig config) {
     ClientHttpRequestInterceptor instrumentationInterceptor = getInterceptor(openTelemetry, config);
 
-    return restClient
-        .mutate()
-        .requestInterceptors(
-            interceptors -> {
-              if (interceptors.stream()
-                  .noneMatch(
-                      interceptor ->
-                          interceptor.getClass() == instrumentationInterceptor.getClass())) {
-                interceptors.add(0, instrumentationInterceptor);
-              }
-            })
-        .build();
+    AtomicBoolean shouldAddInterceptor = new AtomicBoolean(false);
+    RestClient.Builder result =
+        restClient
+            .mutate()
+            .requestInterceptors(
+                interceptors -> {
+                  if (isInterceptorNotPresent(interceptors, instrumentationInterceptor)) {
+                    interceptors.add(0, instrumentationInterceptor);
+                    shouldAddInterceptor.set(true);
+                  }
+                });
+
+    return shouldAddInterceptor.get() ? result.build() : restClient;
+  }
+
+  private static boolean isInterceptorNotPresent(
+      java.util.List<ClientHttpRequestInterceptor> interceptors,
+      ClientHttpRequestInterceptor instrumentationInterceptor) {
+    return interceptors.stream()
+        .noneMatch(interceptor -> interceptor.getClass() == instrumentationInterceptor.getClass());
   }
 
   static ClientHttpRequestInterceptor getInterceptor(

instrumentation/spring/spring-boot-autoconfigure/src/testSpring3/java/io/opentelemetry/instrumentation/spring/autoconfigure/internal/instrumentation/web/RestClientInstrumentationAutoConfigurationTest.java

@@ -87,4 +87,48 @@ void defaultConfiguration() {
                         "otelRestClientBeanPostProcessor", RestClientBeanPostProcessor.class))
                 .isNotNull());
   }
+
+  @Test
+  void shouldNotCreateNewBeanWhenInterceptorAlreadyPresent() {
+    contextRunner
+        .withPropertyValues("otel.instrumentation.spring-web.enabled=true")
+        .run(
+            context -> {
+              RestClientBeanPostProcessor beanPostProcessor =
+                  context.getBean(
+                      "otelRestClientBeanPostProcessor", RestClientBeanPostProcessor.class);
+
+              RestClient restClientWithInterceptor =
+                  RestClient.builder()
+                      .requestInterceptor(
+                          RestClientBeanPostProcessor.getInterceptor(
+                              context.getBean(OpenTelemetry.class),
+                              context.getBean(InstrumentationConfig.class)))
+                      .build();
+
+              RestClient processed =
+                  (RestClient)
+                      beanPostProcessor.postProcessAfterInitialization(
+                          restClientWithInterceptor, "testBean");
+
+              // Should return the same instance when interceptor is already present
+              assertThat(processed).isSameAs(restClientWithInterceptor);
+
+              // Verify only one interceptor exists
+              processed
+                  .mutate()
+                  .requestInterceptors(
+                      interceptors -> {
+                        long count =
+                            interceptors.stream()
+                                .filter(
+                                    rti ->
+                                        rti.getClass()
+                                            .getName()
+                                            .startsWith("io.opentelemetry.instrumentation"))
+                                .count();
+                        assertThat(count).isEqualTo(1);
+                      });
+            });
+  }
 }