Prometheus: Refresh query field metrics on data source change

[s-h-a-d-o-w]

What this PR does / why we need it:
Addresses metrics chooser not refreshing when switching from one Prometheus data source to another (dashboard only).

Which issue(s) this PR fixes:
Fixes #23162

Special notes for your reviewer:
Funnily enough, the reason why this problem didn't occur in Explore was that the query fields got unmounted and "remounted" seemingly unnecessarily. While in Dashboard, they stay mounted.

And so the ideal way to resolve this seemed to me to have PromQueryField refresh the metrics if it sees that languageProvider changed in componentDidUpdate and fix the unnecessary unmounting in Explore, so that the behavior of Dashboard and Explore become consistent in this respect.

As an aside, since storing of props in class fields is generally inadvisable and I didn't see a reason for why it would be necessary here, I refactored this and restructured things a bit so componentDidMount and componentDidUpdate can be more succinct. Of course please let me know if you would rather not have me include this in this PR.

[davkal]

Yeah, we have previously used key changes to wipe query rows because of state issues with the input fields, but it seems like they no longer happen. Since we don't allow ordering or filtering of rows, I think using the index is fine for now (even though it might trip us up later if ordering gets implemented).

The issue itself is nicely fixed by this PR. LGTM

[s-h-a-d-o-w]

even though it might trip us up later if ordering gets implemented

In that case, I would consider a change of the ordering implementation. Keeping a separate index list for an array that is never changed may be great for high performance applications with huge arrays but for a few elements like we have here, I'd go with the improved robustness and readability of simply reordering the query array itself. (Possible caveat: I'm not aware of the bigger picture here yet, so maybe there is something that would make that a bad idea.)