[AzureMonitorExporter] enable nullable reference types

[TimothyMothra]

Library name

Azure.Monitor.OpenTelemetry.Exporter

Please describe the feature.

This issue is to turn on compile time checking of nullables in the Azure Monitor Exporter project.
I'm opening this issue to track and organize PRs.

For this effort, we're turning on nullable enable for the entire project. and then setting nullable disable for offending files.
This will help ensure any new files are compliant while we work to resolve other the individual files.

Related

• https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
• https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/attributes/nullable-analysis