docs: Adding a README for the Tracing module

Hercules Merscherrequested to merge
dist-tracing-readme into master

I missed the documentation for Labkit::Tracing and put Duo Chat to work on that.

The result was quite reasonable, and it was very handy for me to understand what we have available and instrument https://gitlab.com/gitlab-com/gl-infra/observability/tracing-app-ruby. 👌

Edited by Hercules Merscher

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