JsonEncoder currently does not put the formatted message in the json payload. It puts the message and the argument array.
Some of the libs we use, do log using the message format based methods,
which now look strange in our log aggregation system (sometimes we even
see {} as the message with then the actual message as the single member
of the argument array).

Would it be possible to make JsonEncoder configurable to put the formatted message in the json payload with `message` key