[ES|QL] Support avg on aggregate metric double (#130421)

Other major aggregations (min, max, sum, count) are available but avg was not.
This commit adds it in.

docs/changelog/130421.yaml

@@ -0,0 +1,5 @@
+pr: 130421
+summary: Support avg on aggregate metric double
+area: ES|QL
+type: bug
+issues: []

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

@@ -1220,7 +1220,12 @@ public class EsqlCapabilities {
         /**
          * FUSE command
          */
-        FUSE(Build.current().isSnapshot());
+        FUSE(Build.current().isSnapshot()),
+
+        /**
+         * Support avg with aggregate metric doubles
+         */
+        AGGREGATE_METRIC_DOUBLE_AVG(AGGREGATE_METRIC_DOUBLE_FEATURE_FLAG);
 
         private final boolean enabled;
 

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/aggregate/Avg.java

@@ -28,6 +28,7 @@ import java.util.List;
 import static java.util.Collections.emptyList;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.ParamOrdinal.DEFAULT;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.isType;
+import static org.elasticsearch.xpack.esql.core.type.DataType.AGGREGATE_METRIC_DOUBLE;
 
 public class Avg extends AggregateFunction implements SurrogateExpression {
     public static final NamedWriteableRegistry.Entry ENTRY = new NamedWriteableRegistry.Entry(Expression.class, "Avg", Avg::new);
@@ -50,7 +51,7 @@ public class Avg extends AggregateFunction implements SurrogateExpression {
         Source source,
         @Param(
             name = "number",
-            type = { "double", "integer", "long" },
+            type = { "aggregate_metric_double", "double", "integer", "long" },
             description = "Expression that outputs values to average."
         ) Expression field
     ) {
@@ -65,10 +66,10 @@ public class Avg extends AggregateFunction implements SurrogateExpression {
     protected Expression.TypeResolution resolveType() {
         return isType(
             field(),
-            dt -> dt.isNumeric() && dt != DataType.UNSIGNED_LONG,
+            dt -> dt.isNumeric() && dt != DataType.UNSIGNED_LONG || dt == AGGREGATE_METRIC_DOUBLE,
             sourceText(),
             DEFAULT,
-            "numeric except unsigned_long or counter types"
+            "aggregate_metric_double or numeric except unsigned_long or counter types"
         );
     }
 
@@ -105,9 +106,12 @@ public class Avg extends AggregateFunction implements SurrogateExpression {
     public Expression surrogate() {
         var s = source();
         var field = field();
-
-        return field().foldable()
-            ? new MvAvg(s, field)
-            : new Div(s, new Sum(s, field, filter()), new Count(s, field, filter()), dataType());
+        if (field.foldable()) {
+            return new MvAvg(s, field);
+        }
+        if (field.dataType() == AGGREGATE_METRIC_DOUBLE) {
+            return new Div(s, new Sum(s, field, filter()).surrogate(), new Count(s, field, filter()).surrogate());
+        }
+        return new Div(s, new Sum(s, field, filter()), new Count(s, field, filter()), dataType());
     }
 }

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/analysis/AnalyzerTests.java

@@ -2006,7 +2006,7 @@ public class AnalyzerTests extends ESTestCase {
               | stats  avg(x), count_distinct(x), max(x), median(x), median_absolute_deviation(x), min(x), percentile(x, 10), sum(x)
             """, """
             Found 8 problems
-            line 2:12: argument of [avg(x)] must be [numeric except unsigned_long or counter types],\
+            line 2:12: argument of [avg(x)] must be [aggregate_metric_double or numeric except unsigned_long or counter types],\
              found value [x] type [unsigned_long]
             line 2:20: argument of [count_distinct(x)] must be [any exact type except unsigned_long, _source, or counter types],\
              found value [x] type [unsigned_long]
@@ -2028,7 +2028,7 @@ public class AnalyzerTests extends ESTestCase {
             | stats  avg(x), median(x), median_absolute_deviation(x), percentile(x, 10), sum(x)
             """, """
             Found 5 problems
-            line 2:10: argument of [avg(x)] must be [numeric except unsigned_long or counter types],\
+            line 2:10: argument of [avg(x)] must be [aggregate_metric_double or numeric except unsigned_long or counter types],\
              found value [x] type [version]
             line 2:18: argument of [median(x)] must be [numeric except unsigned_long or counter types],\
              found value [x] type [version]

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/analysis/VerifierTests.java

@@ -359,7 +359,7 @@ public class VerifierTests extends ESTestCase {
             error("from test | stats max(max(salary)) by first_name")
         );
         assertEquals(
-            "1:25: argument of [avg(first_name)] must be [numeric except unsigned_long or counter types],"
+            "1:25: argument of [avg(first_name)] must be [aggregate_metric_double or numeric except unsigned_long or counter types],"
                 + " found value [first_name] type [keyword]",
             error("from test | stats count(avg(first_name)) by first_name")
         );

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/aggregate/AvgErrorTests.java

@@ -32,6 +32,13 @@ public class AvgErrorTests extends ErrorsForCasesWithoutExamplesTestCase {
 
     @Override
     protected Matcher<String> expectedTypeErrorMatcher(List<Set<DataType>> validPerPosition, List<DataType> signature) {
-        return equalTo(typeErrorMessage(false, validPerPosition, signature, (v, p) -> "numeric except unsigned_long or counter types"));
+        return equalTo(
+            typeErrorMessage(
+                false,
+                validPerPosition,
+                signature,
+                (v, p) -> "aggregate_metric_double or numeric except unsigned_long or counter types"
+            )
+        );
     }
 }

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/40_tsdb.yml

@@ -707,3 +707,27 @@ to_aggregate_metric_double with multi_values:
   - match: {values.0.1: '{"min":102.44400024414062,"max":195.10000610351562,"sum":297.54400634765625,"value_count":2}'}
   - match: {values.0.2: '{"min":64.0,"max":1456.0,"sum":2139.0,"value_count":4}'}
   - match: {values.0.3: '{"min":123.0,"max":1.9418924E7,"sum":1.9433032E7,"value_count":3}'}
+
+---
+avg of aggregate_metric_double:
+  - requires:
+      test_runner_features: [capabilities]
+      capabilities:
+        - method: POST
+          path: /_query
+          parameters: []
+          capabilities: [aggregate_metric_double_avg]
+      reason: "support avg aggregations with aggregate metric double"
+
+  - do:
+      allowed_warnings_regex:
+        - "No limit defined, adding default limit of \\[.*\\]"
+      esql.query:
+        body:
+          query: 'FROM test2 | STATS avg = avg(agg_metric) | KEEP avg'
+
+  - length: {values: 1}
+  - length: {values.0: 1}
+  - match: {columns.0.name: "avg"}
+  - match: {columns.0.type: "double"}
+  - match: {values.0.0: 4.904761904761905}