[Blazor] Document new Blazor metrics

[javiercn]

          These metrics need to be documented at https://learn.microsoft.com/en-us/aspnet/core/log-mon/metrics/built-in

Originally posted by @JamesNK in #61516 (comment)

[javiercn]

@guardrex hold on for now on this, but I imagine you'll be the one updating the docs?

[guardrex]

Yes, I'll take care of this.

hold on for now

You're suggesting that it won't be for P4 ... it might get pushed to P5?

[javiercn]

It will be in preview4, but we are likely changing names and other stuff as a result of API review (and adding more things)