This replaces the `script` docs for bucket aggregations with runtime
fields. We expect runtime fields to be nicer to work with because you
can also fetch them or filter on them. We expect them to be faster
because their don't need this sort of `instanceof` tree:
a92a647b9f/server/src/main/java/org/elasticsearch/search/aggregations/support/values/ScriptDoubleValues.java (L42)
Relates to #69291
Co-authored-by: James Rodewig <40268737+jrodewig@users.noreply.github.com>
Co-authored-by: Adam Locke <adam.locke@elastic.co>
This commit adds a new single value metric aggregation that calculates
the statistic called median absolute deviation, which is a measure of
variability that works on more types of data than standard deviation
Our calculation of MAD is approximated using t-digests. In the collect
phase, we collect each value visited into a t-digest. In the reduce
phase, we merge all value t-digests, then create a t-digest of
deviations using the first t-digest's median and centroids