Computes client-activity stats for all GRR versions in the DB.

def CountClientVersionStringsByLabel(self, day_buckets):
    """Computes client-activity stats for all GRR versions in the DB."""

    def ExtractVersion(client_info):
      return client_info.last_snapshot.GetGRRVersionString()

    return self._CountClientStatisticByLabel(day_buckets, ExtractVersion)