Add support for offset parameter in Elasticsearch date histograms

[ghost]

What Grafana version are you using?

Version 5.2.1 (commit: 2040f61)

What datasource are you using?

Elasticsearch 5.5.0

What OS are you running grafana on?

Ubuntu 16.04 Server

What did you do?

Split day interval in 8am-8am in UTC time

What was the expected result?

In CTS+0800 time，I need count doc day of day

The picture can count day of day(from 00:00am-00am in UTC time), I want count CTS time from 00:00am-00am(as 16pm-16pm in UTC)

What happened instead?

From elasticsearch document https://www.elastic.co/guide/en/elasticsearch/reference/5.5/search-aggregations-bucket-datehistogram-aggregation.html#offset, I can do

curl -X POST http://172.18.199.240:9200/info_data-*/_search -d '{
   "size" : 0,
   "aggs": {
      "day_of_count": {
         "date_histogram": {
            "field": "@timestamp",
            "interval": "day", 
            "offset": "-8h" 
            
         }
      }
   },
   "query": {
        "range" : {
            "@timestamp" : {
                "gte": "now-5d",
                "lte": "now"
            }
        }
    }
}'

Response:

{
	"took": 18,
	"timed_out": false,
	"_shards": {
		"total": 245,
		"successful": 245,
		"failed": 0
	},
	"hits": {
		"total": 267097,
		"max_score": 0.0,
		"hits": []
	},
	"aggregations": {
		"day_of_count": {
			"buckets": [{
				"key_as_string": "2018-07-13T16:00:00.000Z",
				"key": 1531497600000,
				"doc_count": 17337
			}, {
				"key_as_string": "2018-07-14T16:00:00.000Z",
				"key": 1531584000000,
				"doc_count": 53230
			}, {
				"key_as_string": "2018-07-15T16:00:00.000Z",
				"key": 1531670400000,
				"doc_count": 53922
			}, {
				"key_as_string": "2018-07-16T16:00:00.000Z",
				"key": 1531756800000,
				"doc_count": 53485
			}, {
				"key_as_string": "2018-07-17T16:00:00.000Z",
				"key": 1531843200000,
				"doc_count": 53497
			}, {
				"key_as_string": "2018-07-18T16:00:00.000Z",
				"key": 1531929600000,
				"doc_count": 35626
			}]
		}
	}
}

But I cannot set offset in Grafana? Can and the features or someone tell how can set offset, Thanks

If related to metric query / data viz:

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