This is intended to be used by croslog.
Design doc: go/cros-journald-removal
The boot IDs are stored in /var/log/boot_id.log. Each line corresponds to a single boot.
The example of line (in this case, the boot ID is “12345678901234567890123456789012”)
2020-12-01T00:00:00.000000+00:00 INFO boot_id: 12345678901234567890123456789012