Computes client-activity stats for all client platforms in the DB.

def CountClientPlatformsByLabel(self, day_buckets):
    """Computes client-activity stats for all client platforms in the DB."""

    def ExtractPlatform(client_info):
      return client_info.last_snapshot.knowledge_base.os

    return self._CountClientStatisticByLabel(day_buckets, ExtractPlatform)