I've refactored the stats collection to perform rollups on short-lived requests and added full test coverage for the StatsCollector.

Default rollup window is 1 minute, we can extend it if we want to reduce rows further, but I don't think it'll be necessary.

What's left is:

• Add tests for coderd and wsproxy to ensure app usage produces stats (?)
• Fix TODO about wsproxy auth
• Check correctness of dbauthz implementation

For now, the data looks like this:

SELECT *, session_ended_at - session_started_at AS session_length FROM workspace_app_stats ORDER BY session_started_at;