Use unknown_service as default application name for OpenTelemetry

See gh-38219

Author: Lenin Jaganathan

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/opentelemetry/OpenTelemetryAutoConfiguration.java

@@ -51,7 +51,7 @@ public class OpenTelemetryAutoConfiguration {
 	/**
 	 * Default value for application name if {@code spring.application.name} is not set.
 	 */
-	private static final String DEFAULT_APPLICATION_NAME = "application";
+	private static final String DEFAULT_APPLICATION_NAME = "unknown_service";
 
 	private static final AttributeKey<String> ATTRIBUTE_KEY_SERVICE_NAME = AttributeKey.stringKey("service.name");
 

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/opentelemetry/OpenTelemetryAutoConfigurationTests.java

@@ -95,7 +95,7 @@ void shouldFallbackToDefaultApplicationNameIfSpringApplicationNameIsNotSet() {
 		this.runner.run((context) -> {
 			Resource resource = context.getBean(Resource.class);
 			assertThat(resource.getAttributes().asMap())
-				.contains(entry(ResourceAttributes.SERVICE_NAME, "application"));
+				.contains(entry(ResourceAttributes.SERVICE_NAME, "unknown_service"));
 		});
 	}