vmm: log metrics when the microVM starts
vmm: log metrics when the microVM starts

This allows interested parties to get the process startup time ASAP (in
case the microvM will live for less than 60s).

Signed-off-by: Alexandra Iordache <750ec4522de000f31ad44d64614ad5747f238032@amazon.com>

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