[8.x] Backport to_aggregate_metric_double and sorting (elastic#126438)

* [ES|QL] ToAggregateMetricDouble function (elastic#124595)

This commit adds a conversion function from numerics (and aggregate
metric doubles) to aggregate metric doubles.

It is most useful when you have multiple indices, where one index uses
aggregate metric double (e.g. a downsampled index) and another uses a
normal numeric type like long or double (e.g. an index prior to
downsampling).

* remove old docs

* [ES|QL] Add ToAggregateMetricDouble example (elastic#125518)

Adds AggregateMetricDouble to the ES|QL CSV tests and examples of how to
use the ToAggregateMetricDouble function

* [ES|QL] Fix sorting when aggregate_metric_double present (elastic#125191)

Previously if an aggregate_metric_double was present amongst fields and
you tried to sort on any (not necessarily even on the agg_metric itself)
field in ES|QL, it would break the results.

This commit doesn't add support for sorting _on_ aggregate_metric_double
(it is unclear what aspect would be sorted), but it fixes the previous
behavior.

* drop old style docs again

* add new style docs

* [CI] Auto commit changes from spotless

---------

Co-authored-by: elasticsearchmachine <[email protected]>

Author: limotova

docs/changelog/124595.yaml

@@ -0,0 +1,5 @@
+pr: 124595
+summary: '`ToAggregateMetricDouble` function'
+area: "ES|QL"
+type: enhancement
+issues: []

docs/changelog/125191.yaml

@@ -0,0 +1,5 @@
+pr: 125191
+summary: Fix sorting when `aggregate_metric_double` present
+area: ES|QL
+type: enhancement
+issues: []