Evaluate and document Bootsnap performance in Rails containers for future comparison

Summary

With Rails: bootsnap cache is not performing as to b... (#655 - closed) being largely resolved by Improve cache hit rates with Rails Bootsnap cache (!1546 - merged), we need to do some project maintenance work to ensure we are able to keep this performance enhancement functional and optimized.

Details

We need to get a few tasks done, for future work & validation.

1. Evaluate the best way of determining Rails boot times across containers
   • toolbox
   • webservice
   • sidekiq
2. Document the current expected times
3. Evaluate the best way to determine size and "shape" of Bootsnap cache
   • Such as BOOTSNAP_LOG environment for logging the behaviors
   • Monitoring /srg/gitlab/bootsnap's size in the "base" Rails, and if need be individual downstream containers.
4. Document expectations of Bootsnap
5. Automate the above, if possible