Support dynamic URI building for the upstream service

[dharezlak]

Is your feature request related to a problem? Please describe.
I would like to be able to build upstream service URI based on components of the request. For instance given a request with the path /api/{serviceName} I would like to build the upstream URI using the serviceName piece like so:

Map ports = Map.of("service1", 8080, "service2", 8081);
URI upstreamUri = "http://host:" + ports.get(serviceName) + "/";

Describe the solution you'd like
For the fluent routes (RouteLocatorBuilder) one solution might be to introduce another method e.g. uriBuilder which will have access to request details:

@Bean
public RouteLocator routes(RouteLocatorBuilder builder) {
  return builder.routes()
    .route("dynamic-route", predicate -> predicate.path(/api/**))
    .uriBuilder(request -> {
      String serviceName = "todo"; //extract service name from the reqeust's path
      return "http://host:" + ports.get(serviceName) + "/"
    })
    .build();
}

Describe alternatives you've considered
I couldn't find any alternatives. The closest I could find was the RewritePath GatewayFilter but it allows only for rewriting the path part of the URI.

[dharezlak]

I just stumbled on #276 and I think undocumented AbstractChangeRequestUriGatewayFilterFactory. It looks like it solves the problem stated above.

[ryanjbaxter]

@dharezlak yes I think that is the best solution at the moment. Unfortunately this means the URI of the route is pretty much useless which is kind of nasty. I think we need a cleaner solution to this problem.