Observation Spans are not nested correctly in Webflux

[braunsonm]

Describe the bug
The observations that Spring Security provides are not being properly nested with their parent span when running in Webflux mode. This does not affect servlet.

To Reproduce

1. Run any spring boot application with webflux and spring security to your tracing backend of choice (in my example, Tempo)
2. Make any HTTP call
3. Notice the trace spans are not properly nested

WEBFLUX

Expected behavior
The spans should be nested as they are in Servlet mode. This becomes hard to read in very large or complicated filter chains across multiple services

SERVLET

Sample

There is no need for sample code as your project literally does not need to have any code in it aside from start.spring.io dependencies I listed.

[jzheaux]

Thanks, @braunsonm, for the report. I've scheduled it for our next maintenance release.