[AzureMonitorExporter] fix more nullables with NotNullWhenAttribute

[TimothyMothra]

Towards #34013

This is a follow up to #33870.
In that PR, I had to use the Null-Forgiving operator (!) to declare that the out parameter of a TryGet method was not null.

The better way to do this is to use a null-state attribute.

Changes

• remove null-forgiving operator (!)
• add internal sealed class NotNullWhenAttribute because our project targets NetStardard2.0
  • this is the same thing OTel did in their repo: https://github.com/open-telemetry/opentelemetry-dotnet/blob/1273e83d557cccf0514a8941988c5fdc894afe04/src/OpenTelemetry.Api/Internal/Guard.cs#L52

How to use:

Assume you write the following method:

public bool TryParse(string number, out int? value)

The compiler is not aware that value is not null and gives an error.

Instead you can write:

public bool TryParse(string number, [NotNullWhen(true)] out int? value)

Which indicates that when this method returns true, the value of value is not null.

[azure-sdk]

API change check

API changes are not detected in this pull request.