Improve .NET 8 metrics tests

[JamesNK]

Bug Report

Symptom

The tests added in #4934 need improvement.

• Tests start a web application with socket hard coded to 5000. Test will fail if run in parallel, or another test isn't cleaned up properly, or the machine already has a site running on port 5000
• Tests use Task.Delay to wait for metric result. This slows down test execution and is potentially flaky.

What is the expected behavior?

• Test website uses TestServer and runs in memory.
• Test awaits the metric result to be available. This can be done easily with MetricCollector which has a WaitForMeasurementsAsync method.

What is the actual behavior?

See symptom.

Reproduce

n/a

Additional Context

Add any other context about the problem here.

[JamesNK]

There is documentation here for how to test ASP.NET Core metrics in a correct, stable and efficent manner.

[JamesNK]

Here is another example of testing with TestServer and MetricCollector: https://github.com/dotnet/aspnetcore/blob/334676042dd1b78bad80bf73b5ad2b6133a9519d/src/Middleware/Diagnostics/test/UnitTests/ExceptionHandlerTest.cs#L915-L967

[github-actions]

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[github-actions]

Closed as inactive. Feel free to reopen if this issue is still a concern.