Add paragraph on E2E test metrics dashboards

What does this MR do and why?

Describe in detail what your merge request does and why.

Add paragraph describing custom test metrics setup and usage

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes: https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1036+