This commit instruments the new `RestClient` HTTP client for
observability. Since this client is sharing its HTTP infrastructure with
`RestTemplate` and operates on the same request/response types, this
instrumentation reuses the Observation convention and context.
This choice makes sense since one can build a new `RestClient` instance
using a `RestTemplate` instance, effectively reusing the underlying
configuration.
Closes gh-31114
35fc2df948