Store various metrics (sum,count,etc) as aggregate_metric_double

In 7.11 Elasticsearch added an `aggregate_metric_double` field type:

> Stores pre-aggregated numeric values for metric aggregations. An aggregate_metric_double field is an object containing one or more of the following metric sub-fields: min, max, sum, and value_count.

We could consider doing this for breakdown and span metrics as well. Storage cost might be lower but selfishly/more importantly, it means we can just use an avg aggregation on a field rather than a sum + count aggregation and some Kibana-side post-processing:

> When you run certain metric aggregations on an aggregate_metric_double field, the aggregation uses the related sub-field’s values. For example, a min aggregation on an aggregate_metric_double field returns the minimum value of all min sub-fields.

This field type is also used by rollups I think.

See https://www.elastic.co/guide/en/elasticsearch/reference/master/aggregate-metric-double.html