In many places, the trace context of callbacks was accidentally set to
the client span, not the invocation context. I noticed a hack trying to
work around this. This code fixes all the problems around context. It
also removes some sporadic logging, which was only applied to a few
hooks.
Finally, this adds Brave tests which would have caught the problems
earlier. Notably, there is still more work to do as this will not help
with duplicate instrumentation, which is normal when reactor-netty is
the WebClient's HTTP connector.
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