Prior to this commit, the `DefaultWebClient` would be instrumented for
client observations and would start/stop a `"http.client.requests"`
observation. This would not set this new observation as the current one
in the Reactor context under `ObservationThreadLocalAccessor.KEY`.
This means that potential child observations would not detect it as
their parent; this can happen if the Reactor Netty `HttpClient`
observation is enabled.
This commit ensures that the reactor context is properly populated for
upstream operators.
Fixes gh-29891
7da6e93597