[AzureMonitorExporter] Fix how arrays stored in Activity Tags are serialized.

[TimothyMothra]

I found a bug in the way we serialize some arrays stored Activity Tags.

For example:

using (var activity = activitySource.StartActivity(name: "SayHello", kind: activityKind))
{
    activity?.SetTag("intArray", new int[] { 1, 2, 3 });
}

this would be serialized as key: "intArray" value: "1,2,3".

However,

using (var activity = activitySource.StartActivity(name: "ActivityWithException"))
{
	var eventTags = new Dictionary<string, object>
	{
		{ "intArray", new int[] { 1, 2, 3 } }
	};
	activity?.AddEvent(new("Gonna try it!", DateTimeOffset.Now, new(eventTags)));
}

this is being serialized as key: "intArray" value: "System.Int32[]"

The fix here is to consistently check for Array type and serialize the component items.

Changes

• Add extension method Array.ToCommaDelimitedString() to serialize an Array as a comma delimited string.
  This copies the implementation from AzMonList's TagEnumerationState to use in the TraceHelper.
• Refactor unit tests
• Added a unit test for ActivityEvent.

Screenshot

[azure-sdk]

API change check

API changes are not detected in this pull request.