[Bro] question about event http_stats

[Sen Yang]

Hi All,
I am writing some bro script to analysis http session, and I have some
question about those events

#global http_stats: event(c: connection, stats: http_stats_rec);

the http_stats event, I think it is raised when one http session is
done? But why
the stats$num_requests can be 0?

I noticed in the http.bro, there is one handler for this event but
then annotated.