Feature: select output destination based on message fields

[manuelluis]

Is your feature request related to a problem? Please describe.

If we have one input, for example systemd, and two outputs and we cannot filter the output based on a tag, and we need to send all data to one of the output but only some data to the other output based on some input field.

Describe the solution you'd like

Allow to rewrite tags in filters.

For example in the lua filter we can return the tag.

function cb_print(tag, timestamp, record)
   return code, tag, timestamp, record
end

or without breaking the existing configurations, put at the end and make optional

function cb_print(tag, timestamp, record)
   return code, timestamp, record, tag
end

Describe alternatives you've considered

Duplicate the inputs in the configuration, but in some cases can cause a high overhead

Additional context

[manuelluis]

Since de tags are for a groups of messages, if the filter can change the tag of a message it will implied a lot of big changes, maybe put the tag at message level (tag, timestamp, message) an change all plugins.

Maybe it will more simple a filter that duplicate messages with a different tag, It should require less changes

[FILTER]
    Name   dup
    Match  *
    Tag    new-tag

@edsiper Would this be acceptable?

[nokute78]

Fluentd doesn’t allow rewriting tag at filter by design.
It seems to make dataflow complex.
(see also #291 I tried to do that )

I think what we need is “label” feature and relabel output plugin of fluentd.

https://docs.fluentd.org/v1.0/articles/life-of-a-fluentd-event#labels
https://docs.fluentd.org/v1.0/articles/routing-examples#re-route-event-to-other-label

[edsiper]

New rewrite_tag has landed in GIT master, more details on #293