Remove/hook unused metrics

[alindima]

We have some metrics that are never incremented, for example: sync_response_fails and sync_vmm_send_timeout_count (I assume there are more). We should assess whether it's worth hooking them in the specific Firecracker code or just remove them.

[roypat]

There is some ongoing work to redesign our metrics subsystem to allow for per-device metrics by @sudanl0 at the moment, which will also tackle this issue.