Application Insights and the Azure portal will give you a rich monitoring and diagnostics experience for your web app. It is in preview at this time.
You can get very granular data points or high-level graphs and trends. The historical data can go back as far as 13 months for aggregated data points. You can read more about the data retention policy here.
The Azure portal (the new one at portal.azure.com) gives you a rich UI to interact with the telemetry data from your app. And if you want to customize a blade to view specific data for your app you can do that too.
Azure Web Apps also has built-in monitoring support that you can use. It won't give you the depth that Application Insights does but you should look at this too to see if it will give you the data you need. And of course, you can customize the monitoring blades in the Azure portal to suit your needs.