Make MetricFilter auto-configuration conditional on OncePerRequestFilter

In addition to requiring the Servlet API to be on the classpath,
MetricFilter requires spring-web to also be on the classpath as it
extends OncePerRequestFilter. This commit updates the auto-configuration
for MetricFilter so that it will be disabled in the absence of a 
dependency on spring-web.

Fixes gh-1664

Author: wilkinsona

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/MetricFilterAutoConfiguration.java

@@ -52,7 +52,8 @@
  */
 @Configuration
 @ConditionalOnBean({ CounterService.class, GaugeService.class })
-@ConditionalOnClass({ Servlet.class, ServletRegistration.class })
+@ConditionalOnClass({ Servlet.class, ServletRegistration.class,
+		OncePerRequestFilter.class })
 @AutoConfigureAfter(MetricRepositoryAutoConfiguration.class)
 public class MetricFilterAutoConfiguration {