RFC5424 specifies MSGID as a string (not a number)

[ChezBunch]

The RFC5424 LogFormat implementation expects a u32 message_id, but the RFC5424 specifies MSGID as a string.
From https://datatracker.ietf.org/doc/html/rfc5424:

      MSGID           = NILVALUE / 1*32PRINTUSASCII
...
      PRINTUSASCII    = %d33-126
...
      NILVALUE        = "-"

I don't know if it's possible to change the type of message_id to a string while retaining compatibility with u32.