Prior to this commit, the `RestClient` instrumentation would create and
close observations for HTTP requests, but would not open an observation
scope for the lifetime of the exchange.
This means that custom `ClientHttpRequestInterceptor` and
`ResponseErrorHandler` would not get access to the current observation
scope in case of tracing, possibly leading to missing trace ids in logs.
This commit ensures that an observation scope is managed for the
lifetime of the HTTP exchange.
Fixes gh-33397
50be08480d