ESQL: Add support for exponential_histogram as case value (#138300)

Author: JonasKunz

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

@@ -82,6 +82,19 @@ NULL       | NULL       | NULL
 ;
 
 
+histoAsCaseValue
+required_capability: exponential_histogram_minmax_support
+
+FROM exp_histo_sample 
+ | INLINE STATS p50 = PERCENTILE(responseTime, 50) BY instance, @timestamp
+ | EVAL filteredHisto = CASE(p50 > 0.1, responseTime)
+ | INLINE STATS filteredMax = MAX(filteredHisto) BY instance, @timestamp //MAX is null if the histogram is null
+ | STATS filteredCount = COUNT(filteredMax)
+;
+
+filteredCount:long
+3
+;
 
 ungroupedPercentiles
 required_capability: exponential_histogram_percentiles_support

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/conditional/Case.java

@@ -81,7 +81,8 @@ ConditionEvaluatorSupplier toEvaluator(ToEvaluator toEvaluator) {
             "keyword",
             "long",
             "unsigned_long",
-            "version" },
+            "version",
+            "exponential_histogram" },
         description = """
             Accepts pairs of conditions and values. The function returns the value that
             belongs to the first condition that evaluates to `true`.
@@ -126,7 +127,8 @@ public Case(
                 "long",
                 "text",
                 "unsigned_long",
-                "version" },
+                "version",
+                "exponential_histogram" },
             description = "The value that’s returned when the corresponding condition is the first to evaluate to `true`. "
                 + "The default value is returned when no condition matches."
         ) List<Expression> rest

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/conditional/CaseTests.java

@@ -66,7 +66,6 @@ public class CaseTests extends AbstractScalarFunctionTestCase {
         if (Build.current().isSnapshot()) {
             t.addAll(
                 DataType.UNDER_CONSTRUCTION.stream()
-                    .filter(type -> type != DataType.EXPONENTIAL_HISTOGRAM) // TODO(b/133393): implement
                     .filter(type -> type != DataType.AGGREGATE_METRIC_DOUBLE && type != DataType.DENSE_VECTOR)
                     .toList()
             );