tests: add some assertions to metrics case
tests: add some assertions to metrics case

Compare the jsoned content reading from metrics FIFO with the excepted
group values, to check if the metrics FIFO reading process is as
expected.

Signed-off-by: xiekeyang <ecd7bd01534c8674520527b4a78fc5e108454b6f@gmail.com>

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