[ES|QL] ToAggregateMetricDouble function (#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).

Author: limotova

docs/changelog/124595.yaml

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

docs/reference/query-languages/esql/_snippets/functions/description/to_aggregate_metric_double.md

@@ -192,6 +192,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion INFERENCE_MODEL_REGISTRY_METADATA = def(9_032_0_00);
     public static final TransportVersion INTRODUCE_LIFECYCLE_TEMPLATE = def(9_033_0_00);
     public static final TransportVersion INDEXING_STATS_INCLUDES_RECENT_WRITE_LOAD = def(9_034_0_00);
+    public static final TransportVersion ESQL_AGGREGATE_METRIC_DOUBLE_LITERAL = def(9_035_0_00);
 
     /*
      * STOP! READ THIS FIRST! No, really,

docs/reference/query-languages/esql/_snippets/functions/layout/to_aggregate_metric_double.md

@@ -557,7 +557,6 @@ public static boolean isRepresentable(DataType t) {
             && t != SOURCE
             && t != HALF_FLOAT
             && t != PARTIAL_AGG
-            && t != AGGREGATE_METRIC_DOUBLE
             && t.isCounter() == false;
     }