[] Measure serialization costs cache key generation of getDefinitions

Visualization with Grafana is too complicated to add in the gitlab CI pipeline. No asset in the tests, so it is useless for gitlab CI pipeline but still useful for local dev environment.

So, let's do that:

1. rebase from 2.0.x
2. fix PHPCS issues
3. add a very explicit comment (README?) to tell people this is only useful when grafana is installed and how to set up the test
4. merge

Let's wait pipeline to finsih

Auto merge

Automatically closed - issue fixed for 2 weeks with no activity.