Change import project members API feature category

What does this MR do and why?

• Changes the feature_category for the :id/import_project_members/:project_id API endpoint from experimentation_expansion to projects
  • This change in assignment better reflects the original setup before the API/invite modal was introduced that was in place with the Rails controller assignment where imports were performed previously. Ideally the groupexpansion should not own things like this longterm and should likely follow a path of being handed over in cases like this once the feature is made a permanent part of the product.
• add current measure of the queries performed on the endpoint
• add an exception that the enhanced measuring test now triggers on the API.

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.

Related to #354856 (closed)