Fix Observation Filter docs

`WebMvcMetricsFilter` no longer exists since 3.0 and should be
replaced with `ServerHttpObservationFilter`

See gh-38570

Author: aleksandrserbin

spring-boot-project/spring-boot-docs/src/docs/asciidoc/actuator/metrics.adoc

@@ -757,7 +757,7 @@ TIP: In some cases, exceptions handled in web controllers are not recorded as re
 Applications can opt in and record exceptions by <<web#web.servlet.spring-mvc.error-handling, setting handled exceptions as request attributes>>.
 
 By default, all requests are handled.
-To customize the filter, provide a `@Bean` that implements `FilterRegistrationBean<WebMvcMetricsFilter>`.
+To customize the filter, provide a `@Bean` that implements `FilterRegistrationBean<ServerHttpObservationFilter>`.