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
b267547fb4