Honor timeouts in Zipkin WebClient sender

[mhalbritter]

The WebClientSenderConfiguration contains the auto-configuration for the WebClient based Zipkin sender. The RestTemplate based one honors the timeouts from the ZipkinProperties, while the WebClient based one does not. Unfortunately there is no client agnostic way to set the timeouts (the PR set the timeouts via a ReactorHttpClient, which ties the implementation to Netty).

Investigate if we can make this happen nonetheless, or at least document that the timeouts aren't honored with the WebClient one. Maybe we can use the timeout methods from Reactor somehow to workaround.

[mhalbritter]

I have some changes here: https://github.com/mhalbritter/spring-boot/tree/mh/31496-honor-timeouts-in-zipkin-webclient-sender