AppInsightsTelemetryInitializer.cs

    public class AppInsightsTelemetryInitializer : ITelemetryInitializer
    {

        public void Initialize(Microsoft.ApplicationInsights.Channel.ITelemetry telemetry)
        {

            var userId = HttpContext.Current?.User?.Identity?.GetUserId();

            telemetry.Context.User.AccountId = userId;
            CheckAndAdd(telemetry, "UserId", userId);
            CheckAndAdd(telemetry, "InstanceId", Environment.GetEnvironmentVariable("WEBSITE_INSTANCE_ID"));
        }

        private void CheckAndAdd(Microsoft.ApplicationInsights.Channel.ITelemetry telemetry, string key, string value)
        {
            if (!telemetry.Context.Properties.ContainsKey(key))
            {
                telemetry.Context.Properties.Add(key, value);

            }
        }
    }