Update docs for changing existing metrics
What does this MR do and why?
Updates the docs around changing a metric in line with #408633 (closed) and especially the discussion around #408633 (comment 1366294147).
The aim is:
- Strengthen the wording around metric changes to forbid rather than just discourage them for specific scenarios
- outline the exact parts that should not be changed
Related to #408633 (closed)
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Related to #408633 (closed)