test_metrics: use 'flush_metrics'  from microvm.py
test_metrics: use 'flush_metrics'  from microvm.py

Signed-off-by: Iulian Barbu <3d73465fd48f87db8f58a730f2d3c3f04dff43ae@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker