Sort changed metrics first in metrics reports MR widget

What does this MR do?

For more context, see #260388 (closed)

This MR sorts the existing metrics in the metrics report MR widget so that the changed metrics appear above the unchanged existing metrics. The order of the metrics in the expanded widget list will now be: new metrics first, then changed existing metrics, then unchanged existing metrics, then removed metrics.

This MR also updates the How it works section of the Metrics Reports documentation to explain the order and the meaning of the New and Removed badges that appear next to metrics.

Screenshots

	before	after
order	new, unsorted existing, removed	new, changed existing, unchanged existing, removed
screenshot

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done