Add 3xx redirects to the "unmapped" class of requests for metrics

When Spring Security sends 302 responses to a login page we don't get
any information about the request matching in Spring MVC. Consequently
apps can end up with a lot of counter.status.302.* metrics (where
"*" can be whatever the user sent).

This change treats 3xx the same as 4xx (if it is unmapped it just gets
added to a metric called "unmapped" instead of using the actual request
path).

Fixes gh-2563

spring-boot-samples/spring-boot-sample-web-secure/pom.xml

@@ -19,6 +19,10 @@
 		<main.basedir>${basedir}/../..</main.basedir>
 	</properties>
 	<dependencies>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-actuator</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-security</artifactId>