Which metric should you monitor in Cloud Monitoring to track the number of connections for an application running on App Engine?

Monitoring the number of connections for an application running on App Engine is crucial for understanding its performance and resource utilization. The metric "flex/connections/current" is specifically designed to track the current number of connections to your App Engine instance.

This metric allows you to gauge the active connections that are being handled by your application at any given moment, providing valuable insights into the load and performance characteristics of your application. By monitoring this metric, you can identify trends in connection usage, help in capacity planning, and ensure that your application is scaling appropriately to meet demand.

In contrast, the other metrics listed focus on different aspects of connection handling or apply to different services within Google Cloud. "tcp_ssl_proxy/new_connections" and "tcp_ssl_proxy/open_connections" pertain to TCP and SSL proxy instances, which do not directly reflect the number of connections handled specifically by your App Engine application. "flex/instance/connections/current" tracks instance-level connections in Google Kubernetes Engine rather than App Engine. Hence, "flex/connections/current" is the most relevant and accurate metric to monitor for App Engine applications.