[Idea]: Allow reordering custom metrics sources

[noponoponopo]

Overview of the requested feature

Allow users to reorder custom metrics sources by dragging the handle in Metrics settings.
The new order is persisted in the existing custom metrics configuration and immediately applied to custom metrics displays.

Reason for the request

Users with multiple custom metrics sources currently need to remove and re-add sources to change their display order, so we want to enable custom metrics reordering.

Related Issues

No response

Check List

• I have read the CONTRIBUTING.md and agree to follow it.
• The proposed feature is beneficial to a wide range of users and not just a personal preference.
• This is not a request to add a new runner. Runner requests belong in the Runner Gallery.
• Checked that no similar requests already exist.