logging timestamp and threadcount only

I need to have a load test output for me a simple file showing the threadcount every second or so. I've looked at the standard stat files and can't find quite the right thing; for one thing, I don't want it broken out by test step. I just literally need total threads running as of a timestamp.

I am guessing there is a way to do this with a scripting step but it's not obvious how. Can someone help? Thanks in advance - Edward