Implement session length tracking.
Refactor asserting on metrics to make adding new tests easier.
Add tests for session start on inbound.
Test session timer on inbound messages.
Test session timer on outbound messages.
Add tests for provider metrics.
Add tests for tracking tagpools.
Add tests for track_all_tags.
Add tests for tracking a specific tag.
Add tests for session billing unit timers.
Improve tests for response and session timer expiry times.