Telegraf MQTT JSON Data: Convert String To Integer

My MQTT message is JSON formatted, however the value I need for my InfluxDB comes with questionmarks. I can store it to my InfluxDB when I add the tag_key, but it is still string. Is there any way to change this?

     `{"cmd":"rx","seqno":1214,"EUI":"70B3D5499CF61F5C","ts":1512769317528,"fcnt":2373,"port":2,"freq":868300000,"rssi":-65,"snr":9.8,"toa":46,"dr":"SF7 BW125 4/5","ack":false,"bat":255,"data":"3432"}`

My mqtt.input from Telegraf looks like this:

data_format = "json"
  tag_keys = [

There is no way to do this currently. In the future we will likely add a processor that can do these conversions.

It would be very useful function.