Replace internal Microsoft.Extensions.Metrics with package

[JamesNK]

#46834 adds an internal prototype version of Microsoft.Extensions.Metrics. The types are shared throughout ASP.NET Core with InternalsVisibleTo.

Replace with Microsoft.Extensions.Metrics when available. This work is happening in dotnet/runtime#77514

[JamesNK]

Blocked, waiting on dotnet/runtime#77514 to be completed.

[eerhardt]

Fixed by #48407.

[JamesNK]

@eerhardt Thanks for shepherding this through for preview 5! I was planning to wait for 6, but I didn't think about type name conflicts showing up when the real version was merged.