Tracing: Add feature flag to disable feature

Why are we doing this work

In 15.0, we start removing the code for Tracing. Before deleting code, we should introduce a feature flag to hide the feature. The guarantees we can effectively disable Tracing in %15.0 and allows flexibility to complete the cleanup.

Relevant links

• Feature flag gates should be added for:
• [ ] Settings > Monitor > Tracing sub-section | app/views/projects/settings/operations/_tracing.html.haml
  • Settings > Monitor > Tracing nav | app/views/projects/settings/operations/show.html.haml
  • Monitor > Tracing page (should 404) | app/controllers/projects/tracings_controller.rb
  • Monitor > Tracing nav | lib/sidebars/projects/menus/monitor_menu.rb
• Add info about feature flag in doc/operations/tracing.md
• Remove references to Category:Tracing features in:
  • doc/development/distributed_tracing.md,
  • doc/user/project/settings/index.md

Implementation plan

1. Globally enable a flag called monitor_tracing on dot-com (before the flag exists)
2. Add a feature flag in %15.0, called monitor_tracing which is disabled by default.
   • The documentation should be updated to reflect our plans for removal explicitly, including timeline.
3. For flag rollout, selectively disable the flag for test projects/staging/etc to verify the feature removal/disablement. Globally disable the flag or delete any actor overrides to disable the feature for everyone.
4. Begin code deletions in %15.2 [out of scope of this issue]
   • This gives self-managed users a milestone to quickly regroup if they ended up surprised by the removal, and allows us time to delete the code as we have availability.