ES|QL: Add PRESENT ES|QL function

Add unit tests and documentation for the PRESENT function.

Part of #131069

Author: leontyevdv

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

@@ -56,3 +56,6 @@ The [`STATS`](/reference/query-languages/esql/commands/stats-by.md) command supp
 
 :::{include} ../_snippets/functions/layout/weighted_avg.md
 :::
+
+:::{include} ../_snippets/functions/layout/present.md
+:::

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

@@ -1,16 +1,37 @@
 // PRESENT-specific tests
 
-PRESENT
+PRESENT with no filter
 required_capability: fn_present
 
+// tag::present[]
 FROM employees
 | STATS is_present = PRESENT(languages)
+// end::present[]
 ;
 
 is_present:integer
 1
 ;
 
+PRESENT with grouping
+required_capability: fn_present
+
+// tag::present-by[]
+FROM employees
+| STATS is_present = PRESENT(salary) BY languages
+// end::present-by[]
+;
+ignoreOrder:true
+
+is_present:integer | languages:integer
+1                  | 1
+1                  | 2
+1                  | 3
+1                  | 4
+1                  | 5
+1                  | null
+;
+
 PRESENT with filter
 required_capability: fn_present
 
@@ -57,7 +78,7 @@ is_present:integer
 0
 ;
 
-Not PRESENT with null evaluated field 
+Not PRESENT with null evaluated field
 required_capability: fn_present
 
 FROM employees
@@ -80,30 +101,14 @@ is_present:integer
 1
 ;
 
-PRESENT with grouping
-required_capability: fn_present
-
-FROM employees
-| STATS is_present = PRESENT(salary) BY languages
-| SORT languages
-;
-
-is_present:integer | languages:integer
-1                  | 1
-1                  | 2
-1                  | 3
-1                  | 4
-1                  | 5
-1                  | null
-;
-
 Some fields are not PRESENT with grouping
 required_capability: fn_present
 
 FROM employees
 | WHERE gender IS NULL
 | STATS is_present = PRESENT(gender) BY languages
 ;
+ignoreOrder:true
 
 is_present:integer | languages:integer
 0                  | 4