SpringBoot MetricsRestTemplateCustomizer and LocalHostUriTemplateHandler cannot be used together when building a RestTemplate with RestTemplateBuilder

The changes in #25744 break some usages of LocalHostUriTemplateHandler. The following code fails after updating from 2.4.4 to 2.4.6

restTemplate = restTemplateBuilder.defaultMessageConverters()
        .uriTemplateHandler(LocalHostUriTemplateHandler(context.environment))
        .errorHandler(DefaultResponseErrorHandler())
        .build()

with

java.lang.IllegalArgumentException: RootUri must not be null

at org.springframework.util.Assert.notNull(Assert.java:201)
at org.springframework.boot.web.client.RootUriTemplateHandler.<init>(RootUriTemplateHandler.java:61)
at org.springframework.boot.web.client.RootUriTemplateHandler.withHandlerWrapper(RootUriTemplateHandler.java:96)
at org.springframework.boot.actuate.metrics.web.client.MetricsClientHttpRequestInterceptor.createUriTemplateHandler(MetricsClientHttpRequestInterceptor.java:105)
at org.springframework.boot.actuate.metrics.web.client.MetricsRestTemplateCustomizer.customize(MetricsRestTemplateCustomizer.java:61)
at org.springframework.boot.web.client.RestTemplateBuilder.configure(RestTemplateBuilder.java:634)
at org.springframework.boot.web.client.RestTemplateBuilder.build(RestTemplateBuilder.java:589)

The LocalHostUriTemplateHandler extends RootUriTemplateHandler and so gets the special handling introduced here. However, it does not have the rootUri field set and instead override the getRootUri method, which causes this line to pass a null rootUri

Comment From: wilkinsona

Thanks for the report, @jhorstmann.

Interestingly, the auto-configured TestRestTemplate isn't affected by this despite it using both MetricsClientHttpRequestInterceptor and LocalHostUriTemplateHandler. I think that's due to ordering. The factory overwrites the URI template handler with a LocalHostUriTemplateHandler:

https://github.com/spring-projects/spring-boot/blob/c52143727a9c35b656f2f0d5298344e876785404/spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/TestRestTemplateContextCustomizer.java#L138-L140

This leaves MetricsClientHttpRequestInterceptor without its CapturingUriTemplateHandler so it won't see any URI templates. I'm not sure that this matters for TestRestTemplate as metrics are unlikely to be important for requests made in test code but it does leave things in a bit of an odd state.