[Metrics] Create custom metrics with DI integration

The current documentation for creating a custom metric recommends static meter and counters: https://learn.microsoft.com/en-us/dotnet/core/diagnostics/metrics-instrumentation#create-a-custom-metric

An alternative should be recommended that uses `IMeterFactory`, DI and a custom class that contains the counters. Should discuss how this is more unit testable.

An example worker template app (which uses generic host and DI) can show off writing metrics like this.

Can reuse content from ASP.NET Core docs: https://github.com/dotnet/AspNetCore.Docs/pull/30073