C210K: Multi-region scale test can be executed by anyone

Since we're avoiding full automation of the 10k multi-region scaletesting system, we need internal documentation and a GitHub Action for running it. This will be pretty pricy test that we don't expect customers to run, so no public documentation is needed.

With this guide, a core coder engineer should be able to conduct the test end-to-end.

Steps to document
- [ ] Spinning up the workspace infrastructure
- [ ] Starting load generation
- [ ] How to monitor and prevent failures during the run
- [ ] What to do in the case of failure
- [ ] How to access prometheus data
- [ ] How to convert data into meaningful value