Backoff in the DirectRunner Monitor if no work is Available #18070
Description
When a Pipeline has no elements available to process, the Monitor Runnable will be repeatedly scheduled. Given that there is no work to be done, this will loop over the steps in the transform looking for timers, and prompt the sources to perform additional work, even though there is no work to be done. This consumes the entirety of a single core.
Add a bounded backoff to rescheduling the monitor runnable if no work has been done since it last ran. This will reduce resource consumption on low-throughput Pipelines.
Imported from Jira BEAM-690. Original Jira may contain additional context.
Reported by: tgroh.