Final answer:
Application Insights is the Azure service responsible for monitoring the availability, performance, and usage of web applications, offering telemetry collection and analytics.
Step-by-step explanation:
The service that monitors the availability, performance, and usage of web applications, whether they're hosted in the cloud or on-premises, is Application Insights. This service is part of the Azure Monitor suite and is specifically designed to provide developers with the data they need to understand the performance and usage of their applications. Application Insights collects a wide range of telemetry from web applications such as page views, performance metrics, and exception tracking. It also offers powerful analytic tools to dive deep into the data and uncover insights.