Remove tracing feature via feature flag

What does this MR do and why?

This adds a feature flag, monitor_tracing to allow us to toggle the feature on and off as part of the Deprecation & removal of the feature (#359904 (closed)).

This setting is defaulted to disabled, which means by default the Tracing feature will be turned off. Self-managed users can enable it on their instance with the instructions provided.

Rollout

To ensure the rollout (in this case the removal) is done in stages, the feature flag has already been created and enabled. This ensures when this code goes live that the feature will remain until we explicitly disable it.

This was done here:

Environment	Link to chatops command
Staging	https://gitlab.slack.com/archives/CLM200USV/p1651006730359059
Production	https://gitlab.slack.com/archives/C101F3796/p1651535132053149

How to set up and validate locally

Ensure the tracing feature is removed as per the issue: #359904 (closed)

Enable the Feature flag again for a Project to re-enable it:

Feature.enable(:monitor_tracing, project)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.