Documentation on configuration for new metrics.
Reshuffle configuration parameters into a more sensible order (so less common ones at the end).
Nicer configuration for which tags and tagpools to track (tx @jerith).
Document relying on transports or middleware to provider information on network operators.
Add description for the session billing unit.
Rename operator_metrics to provider_metrics.
Add get_session_time_metric.
PEP8 fix.
Implement simple metrics for transports, providers, tagpools and tags.
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.