Contribute "uri" KeyValue for "/" client requests

Prior to this commit, client HTTP requests performed by `WebClient`
could miss the "uri" KeyValue for simple "/" requests.
This can happen when the baseUri is configured for the client with a
host and a root base path like "https://example.org/"; given the nature
of the `WebClient` API, in these cases, one can perform requests like
this:

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

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

Such a call would contribute a `"none"` value for the `"uri"` KeyValue.

While only templates should be allowed for this keyvalue, we can assume
that requests to `"/"` should be recorded anyway and won't cause
cardinality explosion.

Fixes gh-29879

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientRequestObservationConventionTests.java

@@ -107,6 +107,13 @@ class DefaultClientRequestObservationConventionTests {
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context)).contains(KeyValue.of("client.name", "localhost"));
 	}
 
+	@Test
+	void shouldAddRootUriEvenIfTemplateMissing() {
+		ClientRequestObservationContext context = createContext(ClientRequest.create(HttpMethod.GET, URI.create("https://example.org/")));
+		context.setRequest(context.getCarrier().build());
+		assertThat(this.observationConvention.getLowCardinalityKeyValues(context)).contains(KeyValue.of("uri", "/"));
+	}
+
 	private ClientRequestObservationContext createContext(ClientRequest.Builder request) {
 		ClientRequestObservationContext context = new ClientRequestObservationContext();
 		context.setCarrier(request);