…change (elastic#250963)

## Summary

Resolves elastic#250687.

We are using the index of the items in the list for the grid. This is
not specific enough for a `key` value, because when the list is
reordered, or items are eliminated, the `index` can remain the same (the
0th item is different, but its index is unchanged), and React will
choose not to unmount the existing component.

This change reuses the existing `id` value we already generate, which is
a composite of the index and the metric's name. This makes the list's
render process resilient to item changes where the index value remains
the same.

## Verification

The linked issue lists several cases where this is broken, and I believe
the fix addresses all of them. I have included some recordings. The
issue has detailed repro instructions, so I recommend you review it for
testing. But essentially, if you load any metrics data and perform
filtering or narrowing your ES|QL query to cause the list to re-order
without re-rendering the parent content, you should be able to verify
the fix against `main`.

### Narrowing ES|QL via WHERE clause

![20260129131637](https://github.com/user-attachments/assets/bb00bab1-9b76-4887-92c1-3fbe730b1326)

### Searching for title

![20260129132000](https://github.com/user-attachments/assets/a5b3435b-1289-48ac-9f84-8997d07a2846)

### Checking breakdown -> details matches title

![20260129132042](https://github.com/user-attachments/assets/0a15c0ce-9117-4bc2-8ed0-3124ffaa4e9c)

(cherry picked from commit ddeb3ec)