Implement rate aggregation

[iamazy]

No description provided.

[buinauskas]

@iamazy just spotted this

elasticsearch-dsl-rs/src/search/aggregations/metrics/rate_aggregation.rs

Line 46 in 2c291c3

self.rate.field = Some(field.into());

Isn't field a required field for rate aggregation? Now it's an optional one.

[iamazy]

@iamazy just spotted this

elasticsearch-dsl-rs/src/search/aggregations/metrics/rate_aggregation.rs

Line 46 in 2c291c3

self.rate.field = Some(field.into());

Isn't field a required field for rate aggregation? Now it's an optional one.

When RateAggregation is subaggregation of DateHistogramAggregation, field is not required. Official website gave a example.

GET sales/_search
{
  "size": 0,
  "aggs": {
    "by_date": {
      "date_histogram": {
        "field": "date",
        "calendar_interval": "month"  
      },
      "aggs": {
        "my_rate": {
          "rate": {
            "unit": "year"  
          }
        }
      }
    }
  }
}