[RAM] Create API to draw timelines of alerts on charts

[XavierM]

As a user, I would like to consume an API to draw timelines of Alerts. To be able to do that we will have to use the new field time_range added by this PR. We will use the date histogram aggregations to get the different bucket associated to the timelines of alerts. What is really important in the query is to use hard_bounds to limit the histogram to specified bounds.

GET alert_index/_search
{
  "aggs": {
    "alerts_over_time": {
      "date_histogram": {
        "field": "time_range",
        "fixed_interval": "1m",
        "hard_bounds": {
          "min": "2022-09-21T19:04:00.000Z",
          "max": "2022-09-21T19:10:00.000Z"
        }
      }
    }
  },
  "query": {
    "bool": {
      "filter": [
        {
          
          "range": {
            "time_range": {
              "gt": "2022-09-21T19:04:00.000Z",
              "lt": "2022-09-21T19:10:00.000Z"
            }
          }
        }
      ]
    }
  }
}

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[simianhacker]

This should use BOTH extended_bounds and hard_bounds, without extended_bounds the date_histogram won't draw the empty buckets.

[emma-raffenne]

@XavierM Actionable-observability will need this for the Summary stats widget that will plan to deliver in 8.7.0. Can you confirm you have this on your roadmap on your side?

See elastic/observability-product#190