Revert "Revert "Report USS+PSS from ruby_sampler""

What does this MR do?

This brings back !30374 (merged)

To sidestep the problem with incducing DB IO from outside of the main worker thread, we removed the feature toggle in favor of an environment variable check. The env var is enable_memory_uss_pss.

We will coordinate with an SRE to roll this out via Chef.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

See main description regarding the rollout of this.

Testing of the functionality has already happened on staging.

See also #216597 (closed)