Write a test for doOnCancel branch of WebClientTracingFilter.filter

[trask]

This code in WebClientTracingFilter.filter() is not exercised in any tests:

    .doOnCancel(
        () -> {
          TRACER.onCancel(span);
          TRACER.end(span);
        });

[trask]

Closing, can revisit later if anyone has renewed interest.