Expose default RabbitMQ credentials in the metadata

See gh-6863

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/amqp/RabbitProperties.java

@@ -54,12 +54,12 @@ public class RabbitProperties {
 	/**
 	 * Login user to authenticate to the broker.
 	 */
-	private String username;
+	private String username = "guest";
 
 	/**
 	 * Login to authenticate against the broker.
 	 */
-	private String password;
+	private String password = "guest";
 
 	/**
 	 * SSL configuration.

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/amqp/RabbitPropertiesTests.java

@@ -134,8 +134,8 @@ public class RabbitPropertiesTests {
 	}
 
 	@Test
-	public void usernameDefaultsToNull() {
-		assertThat(this.properties.getUsername()).isNull();
+	public void usernameDefaultsToGuest() {
+		assertThat(this.properties.getUsername()).isEqualTo("guest");
 	}
 
 	@Test
@@ -166,8 +166,8 @@ public class RabbitPropertiesTests {
 	}
 
 	@Test
-	public void passwordDefaultsToNull() {
-		assertThat(this.properties.getPassword()).isNull();
+	public void passwordDefaultsToGuest() {
+		assertThat(this.properties.getPassword()).isEqualTo("guest");
 	}
 
 	@Test