add label to metric enum

Author: limotova

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/AggregateMetricDoubleBlockBuilder.java

@@ -139,20 +139,26 @@ public BlockLoader.IntBuilder count() {
     }
 
     public enum Metric {
-        MIN(0),
-        MAX(1),
-        SUM(2),
-        COUNT(3);
+        MIN(0, "min"),
+        MAX(1, "max"),
+        SUM(2, "sum"),
+        COUNT(3, "value_count");
 
         private final int index;
+        private final String label;
 
-        Metric(int index) {
+        Metric(int index, String label) {
             this.index = index;
+            this.label = label;
         }
 
         public int getIndex() {
             return index;
         }
+
+        public String getLabel() {
+            return label;
+        }
     }
 
     public record AggregateMetricDoubleLiteral(Double min, Double max, Double sum, Integer count) {

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/type/EsqlDataTypeConverter.java

@@ -679,12 +679,12 @@ public static String aggregateMetricDoubleBlockToString(CompositeBlock composite
             for (Metric metric : List.of(Metric.MIN, Metric.MAX, Metric.SUM)) {
                 var block = compositeBlock.getBlock(metric.getIndex());
                 if (block.isNull(index) == false) {
-                    builder.field(metric.name().toLowerCase(), ((DoubleBlock) block).getDouble(index));
+                    builder.field(metric.getLabel(), ((DoubleBlock) block).getDouble(index));
                 }
             }
-            var countBlock = compositeBlock.getBlock(AggregateMetricDoubleBlockBuilder.Metric.COUNT.getIndex());
+            var countBlock = compositeBlock.getBlock(Metric.COUNT.getIndex());
             if (countBlock.isNull(index) == false) {
-                builder.field("value_count", ((IntBlock) countBlock).getInt(index));
+                builder.field(Metric.COUNT.getLabel(), ((IntBlock) countBlock).getInt(index));
             }
             builder.endObject();
             return Strings.toString(builder);