Workaround for metrics tree view selection issue

[tlmii]

This is a workaround for #1262. We should probably be using @key here anyway because of the loop(s). But using it in this way also has the effect of causing blazor to re-render the elements which clears the selection when we want it to.

This doesn't solve the fact that setting _selectedTreeItem doesn't have the effect we want, just for our use case at the moment this seems sufficient to get by.

[JamesNK]

Suggested change

	<FluentTreeItem Class="metrics-tree-item" Text="@meterGroup.Key.MeterName" Data="@meterGroup.Key" title="@meterGroup.Key.MeterName" InitiallyExpanded="true" InitiallySelected="@(_selectedInstrument == null && meterGroup.Key.MeterName == _selectedMeter?.MeterName)">
	<FluentTreeItem @key="@meterGroup.Key" Class="metrics-tree-item" Text="@meterGroup.Key.MeterName" Data="@meterGroup.Key" title="@meterGroup.Key.MeterName" InitiallyExpanded="true" InitiallySelected="@(_selectedInstrument == null && meterGroup.Key.MeterName == _selectedMeter?.MeterName)">

I tested and it is required to stop the first level tree items having the same selection issue.

Unfortunately it introduced a rendering quirk where second level items jump slightly when loaded:

The same thing happened previously when a resource is first selected and the tree is displayed. It wasn't as obvious because it only happened once instead of each time a resource is changed.

Is the rendering jump from items being shown and then web component upgrade?

[tlmii]

It shouldn't be web component upgrade - that issue only really happens when the JS isn't loaded when the elements get rendered.

I think what is happening is when tree items are added as children to tree items, they're marked as nested and then the templating for the tree item runs again and that adds nested to the class. I'd think that would be fast enough we wouldn't notice the rendering, but maybe not.

Another thought is that this could be another issue similar to microsoft/fluentui-blazor#1082 - a similar situation is happening here where FluentTreeItem is setting the class attribute but the underlying tree item is setting it for itself as well (see above). Hrm...

[tlmii]

Alright, I think I found something that works. If I add nested to the ones we know are nested, then it no longer jumps:

[tlmii]

metrics-tree-item wasn't being used anymore.