Scheduled tasks should emit log lines with timings like HTTP requests do #17217
Description
https://element-hq.github.io/synapse/latest/usage/administration/request_log.html?highlight=log%20line#request-log-format is very useful for HTTP requests, but for things that go into a background job, or into a task scheduled by the task scheduler, you don't get such statistics.
A somewhat 'obvious' and likely very useful enhancement would be to have a very similar log line describing completed tasks.