metrics: scaling: cleanup trap should be registered before init phase

[grahamwhaley]

The trap cleanup command is currently done in the run() function in the scaling tests - but, by then we have already done the stats init. This leads to an issue whereby if the previous run has left the stats active in the cluster, or we fail early for any other reason (fail before run()), we end up leaving the stats in place, and never cleaning them up.

We should register the trap cleanup before we do the stats init to try and capture and clean up all errant situations.