Current observation not in scope during WebClient ExchangeFilterFunction execution

[bclozel]

This was discussed with @chemicL in micrometer-metrics/micrometer#5472

Currently, DefaultWebClient executes 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.

ExchangeFilterFunction assertionFilter = (request, chain) -> {
  logger.info("This log will not contain the traceId information");
  return chain.exchange(request);
};

We should defer the execution of filter functions to fix that limitation.