Add inline stats test on agg_metric

Author: limotova

x-pack/plugin/esql/qa/testFixtures/src/main/resources/inlinestats.csv-spec

@@ -3874,3 +3874,24 @@ city:keyword|c:long
 Raleigh     |1
 ;
 
+inlineStatsOnAggregateMetricDouble
+required_capability: inline_stats
+required_capability: aggregate_metric_double_literal_registered
+FROM k8s-downsampled
+| INLINE STATS tx_max = MAX(network.eth0.tx) BY pod
+| SORT @timestamp, cluster, pod
+| KEEP @timestamp, cluster, pod, network.eth0.tx, tx_max
+| LIMIT 9
+;
+
+@timestamp:datetime      | cluster:keyword | pod:keyword | network.eth0.tx:aggregate_metric_double                  | tx_max:double
+2024-05-09T23:30:00.000Z | prod            | one         | {"min":565.0,"max":829.0,"sum":7290.0,"value_count":10}  | 1060.0
+2024-05-09T23:30:00.000Z | prod            | three       | {"min":201.0,"max":582.0,"sum":1794.0,"value_count":6}   | 824.0
+2024-05-09T23:30:00.000Z | prod            | two         | {"min":20.0,"max":190.0,"sum":370.0,"value_count":10}    | 1419.0
+2024-05-09T23:30:00.000Z | qa              | one         | {"min":346.0,"max":356.0,"sum":1765.0,"value_count":5}   | 1060.0
+2024-05-09T23:30:00.000Z | qa              | three       | {"min":605.0,"max":605.0,"sum":605.0,"value_count":1}    | 824.0
+2024-05-09T23:30:00.000Z | qa              | two         | {"min":304.0,"max":1148.0,"sum":8590.0,"value_count":10} | 1419.0
+2024-05-09T23:30:00.000Z | staging         | one         | {"min":263.0,"max":740.0,"sum":5390.0,"value_count":10}  | 1060.0
+2024-05-09T23:30:00.000Z | staging         | three       | {"min":341.0,"max":592.0,"sum":1956.0,"value_count":5}   | 824.0
+2024-05-09T23:30:00.000Z | staging         | two         | {"min":442.0,"max":1011.0,"sum":3850.0,"value_count":7}  | 1419.0
+;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -972,6 +972,11 @@ public enum Cap {
          */
         AGGREGATE_METRIC_DOUBLE_MV_EXPAND(AGGREGATE_METRIC_DOUBLE_FEATURE_FLAG),
 
+        /**
+         * Registering AggregateMetricDoubleLiteral as a NamedWritable.
+         */
+        AGGREGATE_METRIC_DOUBLE_LITERAL_REGISTERED(AGGREGATE_METRIC_DOUBLE_FEATURE_FLAG),
+
         /**
          * Support change point detection "CHANGE_POINT".
          */