PromQL: fix incorrect usage of datetime literals

fixes #138636

Author: felixbarny

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/Literal.java

@@ -219,7 +219,7 @@ public static Literal timeDuration(Source source, Duration literal) {
     }
 
     public static Literal dateTime(Source source, Instant literal) {
-        return new Literal(source, literal, DataType.DATETIME);
+        return new Literal(source, literal.toEpochMilli(), DataType.DATETIME);
     }
 
     public static Literal integer(Source source, Integer literal) {

x-pack/plugin/esql/qa/testFixtures/src/main/resources/k8s-timeseries-promql.csv-spec

@@ -0,0 +1,11 @@
+promql_start_end_step
+required_capability: promql_pre_tech_preview_v1
+TS k8s
+| PROMQL step 5m start "2024-05-10T00:20:00.000Z" end "2024-05-10T00:25:00.000Z" (
+   sum(avg_over_time(network.cost[5m]))
+ );
+
+sum(avg_over_time(network.cost[5m])):double | step:date
+50.25                                       | 2024-05-10T00:20:00.000Z
+;
+