public class CustomTelemetry : ITelemetryInitializer
{
    static string slotName = System.Environment.GetEnvironmentVariable("SlotName");

    public void Initialize(ITelemetry telemetry)
    {
        var requestTelemetry = telemetry as RequestTelemetry;
        if (requestTelemetry == null) return;

        requestTelemetry.Properties["SlotName"]=string.IsNullOrWhiteSpace(slotName)?"Local-Dev":slotName;
    }
}