JSON Decoder destroys existing heka message fields

[maxstepanov]

JSON Decoder will overwrite heka message Fields when it decodes the payload.
I have those fields populated by HTTP Listen Input and i need those afterwards but i also want to decode the JSON payload in HTTP request. Doing so will discard all the fields that already exist.

[maxstepanov]

There is already a #1867 pull request. But it introduces additional configuration of keeping track of remapping which makes things even more complex. I just don't want fields to be lost as it happens now.