Resource usage reports once jobs complete

Proposal

I am opening this request on behalf of a customer, (Internal link) as I think it would be a valuable addition to GitLab.

The original request is quoted here:

It can be quite hard for a user to figure out the right requests and limits to set in their GitLab Runner Kubernetes Executor configuration to support their various CI jobs. Generally you will want a default amount that covers the majority of jobs, and then the ability to override this amount for larger jobs. This functionality exists, but giving the user an awareness as to what they are consuming does not.

We very much would like to see a report at the end of a job run that will surface to the user how much actual ephemeral disk, CPU and RAM they utilized during their job and compare it against the requests and limits that they set so that they have the data needed to tweak these values appropriately.

It would be best to have these resource values over the lifetime of the build containers, as well as helper containers.