Some of these can be cleanly implemented in lib/periodic, others are tasks-specific.

• How long is the period?
• Counters for task start/stop events
• Timestamp for last run
• How long a task took
• Per-task metrics (e.g. num revocations flushed)
• Per-task results (success, db error, ennui)
• For longer-running tasks, maybe a metric to say whether it's currently running or not.