Conditional mutate

[dobesv]

It would be great if I could rename or convert a field conditionally.

Example use cases:

• I get this field level which is a string from some sources but a number from other, so rename level to levelName if it is a string
• I get various fields with the same purpose, like msg, message, log, err.message and I want to rename them to the same field, but only if that field is not already present. e.g. if they provide log and message then I'll just keep both. So a condition on the presence of a field would help.

Possible syntax:

<mutate>
@type rename
@if string
level "levelName"
</mutate>

<mutate>
@type rename
@if unset
log "message"
msg "message"
"err.msg" "message"
</mutate>