Add date function to count calendar date units (hour, day, month, etc.) in a given unit interval.

Author: sidosera

docs/reference/query-languages/esql/_snippets/functions/description/date_unit_count.md

@@ -0,0 +1,18 @@
+{
+  "comment" : "PromQL function definition for Kibana. See https://prometheus.io/docs/prometheus/latest/querying/functions/",
+  "type" : "scalar",
+  "name" : "days_in_month",
+  "description" : "Returns the number of days in the month of the evaluation timestamp.",
+  "signatures" : [
+    {
+      "params" : [ ],
+      "variadic" : false,
+      "returnType" : "scalar"
+    }
+  ],
+  "examples" : [
+    "days_in_month()"
+  ],
+  "preview" : true,
+  "snapshot_only" : false
+}

docs/reference/query-languages/esql/_snippets/functions/examples/date_unit_count.md

@@ -1124,6 +1124,19 @@ Anneke         |Preusig        |1989-06-02T00:00:00.000Z|1989-06-02
 // end::docsDateFormat-result[]
 ;
 
+docsDateUnitCount
+// tag::docsDateUnitCount[]
+ROW dt = to_datetime("2024-02-15T00:00:00.000Z")
+| EVAL days = DATE_UNIT_COUNT("day", "month", dt)
+// end::docsDateUnitCount[]
+;
+
+// tag::docsDateUnitCount-result[]
+dt:date                | days:long
+2024-02-15T00:00:00.000Z | 29
+// end::docsDateUnitCount-result[]
+;
+
 evalDateFormatString
 required_capability: string_literal_auto_casting