Stresses ended well but left stress-ng processes running after finishing

[Inkedstinct]

Hi,

I've just use the snippet from README.md in order to contribute to the common DB.
All went smoothly, only issue is that I noticed increasing latency during the stresses.
Expecting it to drop once the stress process would be finished, the report was sent to the DB and returned successfully, however, the stress-ng processes were left intact running in bg and I had to kill it manually (pgrep "stress-ng" | xargs kill -2).

Even though easily stopped, I wonder if it is intended that all stress survive (bias from one stress-ng config to another). I also guess that all subsequent child stress shall be stopped once the report is completed and sent.