@Amar-Azure-Practice Thanks for reaching out. The HttpQueueLength metric is available on the Microsoft.Web/Serverfarm namespace (App Service Plan).
You can check this in the Azure Portal by navigating to: your App > App Service Plan > Metrics > Select Http Queue Length and the desired Aggregation.
After you analyze available metrics and decide on an appropriate threshold for your app, you can go ahead and configure the Autoscale settings for your resource in the Azure portal.
Hope this helps. Do let us know if you have further questions.
----------
If an answer is helpful, please "Accept answer" and/or "Up-Vote" which might help other community members reading this thread.