chore: Add tracing example

[ikawaha]

issue: #131

see. #130

In continuation from our previous PR (#130), where we addressed certain build issues and streamlined the repository structure, this PR introduces the tracing sample. This sample was separated out to ensure a smoother build process and better clarity.

As discussed, we opted to separate the tracing sample to avoid complications arising from the replace directive. Now that the main repository changes are in place, we believe this sample can be safely merged without any build or dependency issues.

[raphael]

This is great, thank you. One thing I just realized: Goa's release process depends on the top level Makefile of the examples repository, see: https://github.com/goadesign/goa/blob/v3/Makefile#L106. Maybe we should bring the top level Makefile back (and keep the individual Makefiles for each example).

[ikawaha]

Thank you for pointing that out. I've made the necessary adjustments and created a PR to address this concern #138. I've ensured that the top-level Makefile is restored while retaining the individual Makefiles for each example.

By executing the command make GOA_VERSION=v3.13.2, the modules within examples will be regenerated using the specified version of goa, in this case, version v3.13.2. This allows for easy updating and testing of modules in accordance with a specific goa version.

[raphael]

Great, thank you!