ElasticSearch aggregation over inner object

Elastic Stack Elasticsearch
1

In my ES I've a schema type like this:

{
  "index_v1":{
    "mappings":{
      "fuas":{
        "properties":{
          "comment":{
            "type":"string"
          },
          "matter":{
            "type":"string"
          },
          "metainfos":{
            "properties":{
              "department":{
                "type":"string"
              },
              "processos":{
                "type":"string"
              }
            }
          }
        }
      }
    }
  }
}

Shortly, fuas type has two properties comment and matter and an inner (not nested) object metainfos with several properties department and processos.

I'd like to know how many metainfos' fields are informed with its number of occurrences.

Imagine a document doc1 with metainfos: {department: "d1"} and a doc2 with metainfos: {department: "d2", processos: "p1"}.

Then I'd like to get: { department: 2, processos: 1}.

Any ideas?