MVC ProxyExchange does not provide observability of URI template

[MGerriorBB]

Describe the bug Using Spring Cloud 2023.0.3 and Spring Boot 3.2.7, it appears that the micrometer observability for the MVC version of ProxyExchange does not properly support URI templates. Looking at actuator at /actuator/metrics/<project>.http.client.requests the URI tag is always "none". I'm not sure if it's due to use of RestTemplate directly https://github.com/spring-cloud/spring-cloud-gateway/blob/main/spring-cloud-gateway-mvc/src/main/java/org/springframework/cloud/gateway/mvc/ProxyExchange.java#L170 as opposed to RestTemplateBuilder per https://docs.spring.io/spring-framework/reference/integration/observability.html#observability.http-client.resttemplate ?

Sample Won't be able to provide the exact code, but we use UriComponentsBuilder which may be causing issues? Something like:

        return proxyExchange.uri(
            UriComponentsBuilder.fromHttpUrl("http://service.internal.com")
                .path("/v1/documents/{documentId}")
                .build(Map.of("documentId", "<documentId from PathVariable>")
        ).get();

[spencergibb]

I don't know if proxy exchange ever provided that.

[MGerriorBB]

I take back my earlier comment about using RestTemplate as opposed to RestTemplateBuilder, after doing some more digging, I believe the RestTemplateBuilder with Observability should have been injected here: https://github.com/spring-cloud/spring-cloud-gateway/blob/4a2c6d836fc8cf93afc371c06f4bdc8a6d4a0965/spring-cloud-gateway-mvc/src/main/java/org/springframework/cloud/gateway/mvc/config/ProxyResponseAutoConfiguration.java#L63

Given that, I believe this could be solved by using exchange with urivariables here https://github.com/spring-cloud/spring-cloud-gateway/blob/4a2c6d836fc8cf93afc371c06f4bdc8a6d4a0965/spring-cloud-gateway-mvc/src/main/java/org/springframework/cloud/gateway/mvc/ProxyExchange.java#L359 .

Does that sound like the right approach? Of course we'd need to add some setters on ProxyExchange like:

proxyExchange.uri("http://service.internal.com")
    .path("/v1/documents/{documentId}")
    .uriVariables(Map.of("documentId", "foo"))
    .get();

Is the project still actively maintained? If so I can try to open a PR, though I admit I'm not quite a spring wizard yet.