Get http.route while using spring-cloud-gateway

[123liuziming]

Is your feature request related to a problem? Please describe.

While using spring-cloud-gateway, we always get null http.route because spring-cloud-gateway does not put the HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE in exchange object, which is inconvenient for the user because we group metrics by this attribute.
If we get the http.route, we may show the overall metrics like the pic below:

or there is only an overall metrics item, which is unfriendly to users.

Describe the solution you'd like

Add a spring-cloud-gateway instrumentation to get the matched route as the http.route attribute:

1. The httpRouteGetter returns a string in the following format Route@${route_id}
2. Record the route's detail information in Spring Webflux's span attributes

Describe alternatives you've considered

Record spring-cloud-gateway's route info in spring webflux's HttpServerRouteGetter directly. It can result in coupling between modules.

Additional context

No response