Prior to this commit, the `RestTemplate` `ClientHttpObservation` would
be started before the request object is available. While this would also
measure the connection estalishment for some HTTP client libraries, this
arrangement is incompatible with a tracing approach where the request
must be available to propagate information through the request headers.
This commit ensures that the observation only starts when the request is
available.
Fixes gh-29234
a18c692b53