Emulate production load on Staging Ref

We need to increase the background traffic noise on Staging Ref to catch when error rate goes above SLIs in monitoring.

• Select the tool: GPT may not be the best suited for this purpose as it's mostly used to run the test to get results and not to do a constant load. Perhaps it's better to explore other options like webcrawlers (traffic-generator, cmbr, etc) or use both crawler and GPT.
• Analyse what endpoints should be loaded and how many RPS should be used for Staging Ref using Production data and scaling it down
• Clarify what test data should be set up on the environment
• Set up pipeline to emulate users activity
• Verify that alerts are working - depends on https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14047
• Explore how to make performance set up (seed test data, trigger test run) automated if Staging Ref is rebuilt