Update DORA metrics Lead time for Changes definition

What does this MR do and why?

Docs say we measure time to successfully deliver a commit into production while in fact we measure merge request delivery time. This is different because MRs can have multiple commits. E.g.:

MR1 has 1 commit. Delivered in 5 minutes
MR2 has 1 commit. Delivered in 7 minutes
MR3 has 100 commits. Delivered in 50 minutes.

With current logic "time to deliver a MR into production" lead time for changes = 7 minutes With current logic "time to deliver a commit into production" lead time for changes = 50 minutes

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.