tests: randomize metric name
tests: randomize metric name

This makes sure we don't reuse the same name in tests running in
parallel.

Change-Id: I5fc22e37689f4397ae0c06d347a70ca1adec618a

sileht/gnocchi,idegtiarov/gnocchi-rep,idegtiarov/gnocchi-rep,idegtiarov/gnocchi-rep,leandroreox/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi