Prior to this commit, the `DefaultWebClient` would execute the configured
`ExchangeFilterFunction` as the reactive pipeline is assembled during
subscription. This means that if imperative code is executed in a filter
function, it won't be aware of the current observation through the local
scope.
For example, when automatic context propagation is enabled for Reactor
operators, the logger MDC will not know about the current
traceId/spanId.
This commit ensures that client filter functions execution is deferred
during the actual client exchange.
Fixes gh-33559
776811bdb8