Make boot metrics

[cooperlees]

Let's start with logging X slowest units at boot

• Will allow admins to know what's the slowest at boot to fix

i.e.

• systemd-analyze blame
  • See how CLI does it ... hopefully via dbus/varlink and do naively

3min 24.159s dnf5-automatic.service
1min 43.050s cpe_chef.service
     16.210s sys-module-fuse.device
     15.809s dev-ttyS0.device

Put into struct in common unit (probably a float of seconds ...)

Allow config to say only x. Something like:

[boot]
enabled = true
num_slowest_units = 5

[noxiouz]

yes, it uses DBus API (also there is a Varlink API for ListUnits now), But there is a big chunk of custom logic on top of that:
https://github.com/systemd/systemd/blob/69646ac0e2eb1e20fac4ebada42cb043c325edc8/src/analyze/analyze-time-data.c#L277