appropriately set the generation method of the compute grouping type

Created by: coury-clark

Setting the appropriate generation_method when a grouping type insight comes through.

Test plan

Created an insight with the following input:

{
  "input": 
  {
    "dataSeries":
      {
        "groupBy": "LANG",
        "generatedFromCaptureGroups": true,
        "query": "Messages",
        "label": "abc",
        "repositoryScope": {
          "repositories": ["github.com/sourcegraph/sourcegraph"]
    		},
        "timeScope": {
          "stepInterval": {
            "unit": "MONTH",
            "value": 1
          }
        },
        "options": {
          "label": "label me",
					"lineColor": "blue"
        }
      },
    "options": {
      "title": "testing compute group insight 2"
    }, 
    "viewControls": {
      "filters": {
        
      },
      "seriesDisplayOptions": {
        
      }
    }
  }
}