Checkout performance benchmarking

[senadir]

This is an epic task that is meant to:

• Research what kind of metrics we should be tracking (locally in CI, globally in merchants stores).
• Implement tracking locally/globally.

We're not yet to act on the results of this benchmarking yet, just collecting it.

Prior work for local GB tracking:

• https://codehealth.vercel.app/
• https://github.com/youknowriad/codehealth
• Automation: Track Gutenberg performance metrics over time. WordPress/gutenberg#33694

GB work only tracks direct users (in our case merchants), we're interested in seeing the experience of customers as well.

[ralucaStan]

Closing this epic, as we will most likely start a separate performance effort later this year.