Reads flow log entries of a given flow using given query options.

def ReadFlowLogEntries(self,
                         client_id,
                         flow_id,
                         offset,
                         count,
                         with_substring=None):
    """Reads flow log entries of a given flow using given query options."""
    entries = sorted(
        self.flow_log_entries.get((client_id, flow_id), []),
        key=lambda e: e.timestamp)

    if with_substring is not None:
      entries = [i for i in entries if with_substring in i.message]

    return entries[offset:offset + count]