Returns number of flow log entries of a given flow.

def CountFlowLogEntries(self, client_id, flow_id, cursor=None):
    """Returns number of flow log entries of a given flow."""

    query = ("SELECT COUNT(*) "
             "FROM flow_log_entries "
             "FORCE INDEX (flow_log_entries_by_flow) "
             "WHERE client_id = %s AND flow_id = %s ")
    args = [db_utils.ClientIDToInt(client_id), db_utils.FlowIDToInt(flow_id)]

    cursor.execute(query, args)
    return cursor.fetchone()[0]