Fine performance metrics: Meter wasted partial compute time after losing a worker

[crusaderky]

• Part of Fine performance metrics meta-issue #7665
• Blocked by Fine performance metrics: Store data on the Scheduler #7666
• Blocked by Fine performance metrics: Meter currently-executing tasks #7677
• Complements Fine performance metrics: Meter task re-execution after losing a worker #7676

When we lose a worker, any time spent partially executing a task is lost. The task transitions back from processing to released on the scheduler, so that it may be executed somewhere else.

When the scheduler receives metrics from the heartbeat (#7666), it normally immediately forgets which worker they come from. It should make an execption for currently-running (#7677) and keep track of which worker they come from.

When a worker dies, it should subtract all currently-running time for the worker and reclassify it as ("execute", <prefix>, "killed-worker", "seconds").

Additionally, it should add to this measure all tasks that were currently executing as of the last received heartbeat, minus those that completed in the meantime, multiplied by the time between the last heartbeat and the worker death.