[AzureMonitorExporter] cleanup nullable in Exporter classes and Transmitter

[TimothyMothra]

Towards #34013

Changes

• remove #nullable disable in Exporter classes and resolve issues
• remove #nullable disable in Transmitter class and resolve issues

Notes

The biggest change here is that the AzureMonitorResource in the Exporter could be null.
This is provided to the TelemetryItem constructor which is eventually used to set Tags.
The Tags were already checking for null so I think this change is safe.

azure-sdk-for-net/sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src/Customizations/Models/TelemetryItem.cs

Lines 99 to 102 in b6eb01b

	private void SetResourceSdkVersionAndIkey(AzureMonitorResource? resource, string instrumentationKey)
	{
	InstrumentationKey = instrumentationKey;
	Tags[ContextTagKeys.AiCloudRole.ToString()] = resource?.RoleName;

[azure-sdk]

API change check

API changes are not detected in this pull request.

[rajkumar-rangaraj]

LGTM.

@vishweshbankwar Could you please take a look and confirm.