Spring HTTP client observations miss uri keyvalue for "/" requests

Affects: Spring Framework 6.0.4

Problem

After upgrading including webflux version 6.0.4, the URI tag from the http.client.requests metrics are always none when making web client calls.

After the web client is called several times, the output from the metrics/http.client.request endpoint looks like this

Steps to reproduce

1. Create a Srping Boot 3.0.2 application with reactive web.
2. Add org.springframework.boot:spring-boot-starter-actuator and io.micrometer:micrometer-registry-prometheus dependencies.
3. Create a simple get endpoint that calls a webclient and perform several get requests.
4. Check the metrics/http.client.request endpoint, look at the uri tag that has value "none"

I created a simple demo project to reproduce the issue here.

Comment From: bclozel

The behavior changed there to only record the uri key value if a uri template is being used to avoid cardinality explosion.

Let me check with your sample.

Comment From: bclozel

I think this can be summarized with the following call:

WebClient client = WebClient.builder()
    .observationRegistry(registry)
    .baseUrl("https://example.org/")
    .build();

String response = client.get().retrieve().bodyToMono(String.class).block();

In this case, the client call doesn't use the uri(String uri, Object... uriVariables) method, so no URI template is being used. This behavior is consistent with the client usage. Using the following code snippet instead should make the uri tag available:

WebClient client = WebClient.builder()
    .observationRegistry(registry)
    .baseUrl("https://example.org")
    .build();

String response = client.get().uri("/").retrieve().bodyToMono(String.class).block();

Requiring developers to switch to this variant for this case doesn't make sense, so we should add a special case for "/" templates and record them as KeyValue no matter how they were produced.