example-msg-generator: support fast message generation

[vatsal-encs]

fixes #3241

[kira-syslogng]

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[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
"add to whitelist" add author of a Pull Request to whitelist (globally, be careful, it means this user can trigger kira for any PR)
do nothing -> CI won't start)

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
"add to whitelist" add author of a Pull Request to whitelist (globally, be careful, it means this user can trigger kira for any PR)
do nothing -> CI won't start)

[Kokan]

@kira-syslogng ok to test

[kira-syslogng]

Build SUCCESS

[furiel]

@vatsal-encs I mentioned a reference to an e2e test in the original issue. Could you update that too? From now freq="0" should work. It would implicitely test your code too.

[kira-syslogng]

Build SUCCESS

[kira-syslogng]

Build SUCCESS

[kira-syslogng]

Build SUCCESS

[kira-syslogng]

Build SUCCESS

[furiel]

Originally, I was thinking that we could do this in a timerless fashion. Even with freq=0, every messages are generated with a timer in different loops of the event loop. Which feels unnecessary, and maybe slower than not having timers at all.

On the other hand I always forget example-msg-generator is a simple source, and not a threaded source. So quite some work would needed to achieve that. Improved management of free_to_send, and introducing log-fetch-limit, maybe others? Also, even with null-timers, this source is very fast, so I would not open the scope now.

Approve from my side.