Aggs: use time_zone in date_histogram to format bucket keys

[cbuescher]

Follow up issue to the date_histogram clean up in #9062. Since we have only one time_zone option now, we could use this to print the keys of buckets in a date_histogram that uses other time zone than UTC to the specified time zone.

More details about how this could look like here: #9062 (comment)

[jpountz]

+1

[cbuescher]

I'm working on a solution with minimal code changes that would only require a setter for the time zone in the DateTime formatter and a small change in the DateHistogramParser. I ran some local tests, a query like

GET /dates/docs/_search?search_type=count 
{
  "aggs" : {
    "date_agg" : {
      "date_histogram": {
        "field": "date",
        "interval": "month",
        "time_zone": "Europe/Amsterdam"
      }
    }
  }
}

would now get results like:

"aggregations": {
      "date_agg": {
         "buckets": [
            {
               "key_as_string": "2014-03-01T00:00:00.000+01:00",
               "key": 1393628400000,
               "doc_count": 4
            },
            {
               "key_as_string": "2014-06-01T00:00:00.000+02:00",
               "key": 1401573600000,
               "doc_count": 2
            },
            [...]
         ]
      }
   }

The nice thing is that key_as_string is still a UTC date makes more sense in terms of the specified time zone because it seemingly starts at midnight but the zone is appended. This also works nicely with DST (as demostrated above).

Without specifying the time_zone the result would be something like:

"aggregations": {
      "date_agg": {
         "buckets": [
            {
               "key_as_string": "2014-03-01T00:00:00.000Z",
               "key": 1393632000000,
               "doc_count": 4
            },
            {
               "key_as_string": "2014-06-01T00:00:00.000Z",
               "key": 1401580800000,
               "doc_count": 2
            },
            [...]
         ]
      }
   }

Note that this doesn't affect custom formatting like:

"format" : "YY-MM-dd#HH-mm-ss#Z"  
-->
{
        "key_as_string": "14-03-01#00-00-00#+0100",
        "key": 1393628400000,
        "doc_count": 4
},

Also, the time zone can entirely be dropped with custom formatting so it looks like local time, but the key is still UTC:

"format" : "YY-MM-dd#HH:mm:ss"  
-->
{
        "key_as_string": "14-03-01#00:00:00",
        "key": 1393628400000,
        "doc_count": 4
},

[cbuescher]

I wonder if this needs to be configurable because it is a nice default behaviour and one can always specify a format that drops the time zone using custom formatting. I will open a PR with my current changes for discussion.

[jpountz]

@cbuescher The behaviour described above looks great to me. Also +1 to not introduce additional settings.