Add mainApplicationClass tag to startup step

This commit adds a new tag to the `"spring.boot.application.starting"`
startup step.

Closes gh-23162

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -312,7 +312,7 @@ public class SpringApplication {
 		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
 		configureHeadlessProperty();
 		SpringApplicationRunListeners listeners = getRunListeners(args);
-		listeners.starting();
+		listeners.starting(this.mainApplicationClass);
 		try {
 			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
 			ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplicationRunListeners.java

@@ -49,8 +49,12 @@ class SpringApplicationRunListeners {
 		this.applicationStartup = applicationStartup;
 	}
 
-	void starting() {
-		doWithListeners("spring.boot.application.starting", SpringApplicationRunListener::starting);
+	void starting(Class<?> mainApplicationClass) {
+		doWithListeners("spring.boot.application.starting", SpringApplicationRunListener::starting, (step) -> {
+			if (mainApplicationClass != null) {
+				step.tag("mainApplicationClass", mainApplicationClass.getName());
+			}
+		});
 	}
 
 	void environmentPrepared(ConfigurableEnvironment environment) {