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.

[LDagnachew]

Not sure if this is still needed, but I'm fine with taking a look at this issue if needed @roypat.

[roypat]

Not sure if this is still needed, but I'm fine with taking a look at this issue if needed @roypat.

So there are definitely a ton of metrics that are never incremented/set anywhere in the code, and cleaning that up is still very much wanted I think! It's just a bit tricky to find them, because an IDE won't be able to find them as dead code (generally, they are still formally "used" in constructors and aggregation functions, or random tests), so it's literally a matter of going through each of them and checking whether any of the uses are actually incrementing/setting the metric from production code, and if none exist, deleting it.

[mdhaduk]

Hi @roypat, I submitted a draft PR and would love some comments on my work. Feel free to take a look at your own convenience. Thanks!

Note: I'm aware I'm failing tools/devtool checkstyle; I will rebase and fix this before submitting my PR