Summ of two buckets

Hi.
I'm using Kibana to show a particular metric for data collected from a local SIP device. My data is indexed like this:

{
  "_index": "calls",
  "_type": "_doc",
  "_id": "0ysOqHgBtkTNQGR8TH85",
  "_version": 1,
  "_score": null,
  "_source": {
    "timestamp": "1617727201",
    "gateway": "gate1.usuario1.01",
    "@timestamp": "2021-04-06T16:40:10.094Z",
    "@version": "1",
    "activecalls": 14,
    "log": {
      "file": {
        "path": "/var/log/metricas/outbound.activecalls"
      }
    },
    "@fecha_exe": "2021-04-06T16:40:01.000Z",
    "message": "2021 Apr 6 12:40:01;1617727201;usuario1,gate1.usuario1.01,14;usuario1,gate2.usuario1.02,30;user2,gw.01,0;user2,gw.02,7",
    "data": "usuario1,gate1.usuario1.01,14",
    "fields": {
      "type": "outbound-activecalls"
    },
    "fecha": "2021 Apr 6 12:40:01",
    "usuario": "usuario1",
    "input": {},
    "tags": [
      "beats_input_codec_plain_applied"
    ],
    "agent": {
      "name": "eksdata"
    },
    "ecs": {},
    "host": {
      "name": "eksdata"
    }
  },
  "fields": {
    "@timestamp": [
      "2021-04-06T16:40:10.094Z"
    ],
    "@fecha_exe": [
      "2021-04-06T16:40:01.000Z"
    ]
  },
  "sort": [
    1617727201000
  ]
}

So, the important fields are : "usuario", "activecalls", "gateway" and "fields.type"
I need to show a metric wich has the summ of "activecalls" (avg) for a particular user, including the "inbound" and "outbound" type of calls for all the gateways......so far i was able to use a "metric" type visualization but i only can get the values per "inbound" and "outbound".. is there a way to summ this two values in a metric visualization???
These are screenshots for the configuration i used for the latter:

img1 img2 img3

And this is the result:

But i need those two values added.

is there a way to have this?.. even if i can do it with a Time Series Visualization...
Thanks!
Ricardo

What about a scripted field?

Thanks @AClerk for the idea....
I'm not an "advance" user in Kibana and Elasticsearch, so i wonder how can i use the "scripted field" in the visualization... let's asumme i have the two values as indicated in the visualization metric above... how can I use the scripted field inside this metric visualization? or maybe the approach is other?
Thanks!