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

[stirby]

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