Help Center session count & average session duration metrics

Add session-level metrics to Help Center analytics \u2014 specifically number of sessions and average session duration, similar to what Zendesk's Self-service dashboard provides.

Current Help Center analytics cover unique visitors, top pages, traffic sources, devices, and browsers, but there's no session-based reporting today. Teams migrating from Zendesk rely on these numbers for their existing self-serve support reporting and need parity to switch over confidently.

Raised during an active enterprise trial migrating from Zendesk (iClassPro).