Merge "bootstat: Calculate and log the time_since_last_boot metric." into nyc-mr1-dev

void RecordBootComplete() {

  BootEventRecordStore boot_event_store;

  BootEventRecordStore::BootEventRecord record;

  time_t uptime = bootstat::ParseUptime();

  time_t current_time_utc = time(nullptr);

  if (boot_event_store.GetBootEvent("last_boot_time_utc", &record)) {

    time_t last_boot_time_utc = record.second;

    time_t time_since_last_boot = difftime(current_time_utc,

                                           last_boot_time_utc);

    boot_event_store.AddBootEventWithValue("time_since_last_boot",

                                           time_since_last_boot);

  }

  boot_event_store.AddBootEventWithValue("last_boot_time_utc", current_time_utc);

  // The boot_complete metric has two variants: boot_complete and

  // ota_boot_complete.  The latter signifies that the device is booting after