I'm happy to have this exposed as "Solution". Perhaps with a shorter title.😀I edited the original.
I updated it with frac_digits(6); to get microseconds as the logger seems to support that.
Output from F5s does NOT appear to be RFC5424 compliant with this change. For example, I see an ascii level indicator which rfc5424 does not include. I'm looking into further validation.
The F5s should really have built in support to log in rfc5424 with short names, timezone and milli/microsecond information. Supporting only fqdn and only rfc3164 is pretty lame in this day and age.
There are a few F5 KB articles that recommend adding udp() to syslog include. udp() has been deprecated for a long time now. Any KB references that include that should be updated.
BIG-IP 15.1.10.3 includes syslog-ng 3.8.1 which no longer has reliable online documentation that I can find. If F5 is going to continue to ship this old version, they should at least post the documentation online. syslog-ng 3.8.1 shipped on Aug 19, 2016:
https://github.com/syslog-ng/syslog-ng/releases/tag/syslog-ng-3.8.1