[8.x] [ES|QL] ToAggregateMetricDouble function (elastic#124595) (elastic#125172)

* [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).

* modify docs and factory constructors

Author: limotova

docs/changelog/124595.yaml

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

docs/reference/esql/functions/description/to_aggregate_metric_double.asciidoc

@@ -196,6 +196,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion ML_INFERENCE_DEEPSEEK_8_19 = def(8_841_0_09);
     public static final TransportVersion ESQL_SERIALIZE_BLOCK_TYPE_CODE = def(8_841_0_10);
     public static final TransportVersion ESQL_FAILURE_FROM_REMOTE = def(8_841_0_11);
+    public static final TransportVersion ESQL_AGGREGATE_METRIC_DOUBLE_LITERAL = def(8_841_0_12);
 
     /*
      * STOP! READ THIS FIRST! No, really,