Merging buckets of aggregation

[Hari_Prasad]

I am trying to merge buckets from a parent aggregation. I am not sure if thats possible. Please help.

I have an aggregation query as below

GET log-2019.07.20/_search
{
  "size": 0,
  "aggs": {
    "buckets": {
      "date_histogram": {
        "field": "@timestamp",
        "interval": "5m",
        "format": "mm"
      }
    }
  }
}

and the output is like

.
.
.
"aggregations" : {
    "buckets" : {
      "buckets" : [
        {
          "key_as_string" : "00",
          "key" : 1563840000000,
          "doc_count" : 3535
        },
        {
          "key_as_string" : "05",
          "key" : 1563840300000,
          "doc_count" : 3556
        },
.
.
.

What I am trying to do is run a term aggregation on the key_as_string field but its always showing empty bucket.

Is there any way to combine buckets ??

Thank you