Is single character log level. As a rule is is either @verb{|I|}
(informational message), or @verb{|E|} (error message).
@item DATETIME
- UTC datetime in RFC 3339 @verb{|2006-01-02T15:04:05.999999999Z|} format.
+ UTC datetime in @url{https://tools.ietf.org/html/rfc339, RFC 3339}
+ @verb{|2006-01-02T15:04:05.999999999Z|} format.
@item SD
- Structured data as in RFC 5424.
+ Structured data as in @url{https://tools.ietf.org/html/rfc5424, RFC 5424}.
@item MSG
Arbitrary UTF-8 encoded text data.
@end table