ES|QL: enable EXPLAIN (snapshot only) (elastic#129526)

Author: luigidellaquila

x-pack/plugin/esql/qa/server/src/main/resources/explain.csv-spec

@@ -33,12 +33,12 @@ query
     ;
 
 sourceCommand
-    : explainCommand
-    | fromCommand
+    : fromCommand
     | rowCommand
     | showCommand
     // in development
     | {this.isDevVersion()}? timeSeriesCommand
+    | {this.isDevVersion()}? explainCommand
     ;
 
 processingCommand
@@ -239,11 +239,11 @@ commandOption
     ;
 
 explainCommand
-    : EXPLAIN subqueryExpression
+    : DEV_EXPLAIN subqueryExpression
     ;
 
 subqueryExpression
-    : OPENING_BRACKET query CLOSING_BRACKET
+    : LP query RP
     ;
 
 showCommand

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.tokens

@@ -9,12 +9,12 @@ lexer grammar Explain;
 //
 // Explain
 //
-EXPLAIN : 'explain'           -> pushMode(EXPLAIN_MODE);
-
+DEV_EXPLAIN : {this.isDevVersion()}? 'explain'           -> pushMode(EXPLAIN_MODE);
 
 mode EXPLAIN_MODE;
-EXPLAIN_OPENING_BRACKET : OPENING_BRACKET -> type(OPENING_BRACKET), pushMode(DEFAULT_MODE);
+EXPLAIN_LP : LP -> type(LP), pushMode(DEFAULT_MODE);
 EXPLAIN_PIPE : PIPE -> type(PIPE), popMode;
+
 EXPLAIN_WS : WS -> channel(HIDDEN);
 EXPLAIN_LINE_COMMENT : LINE_COMMENT -> channel(HIDDEN);
 EXPLAIN_MULTILINE_COMMENT : MULTILINE_COMMENT -> channel(HIDDEN);

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.g4

@@ -23,6 +23,9 @@ FROM_COMMA : COMMA -> type(COMMA);
 FROM_ASSIGN : ASSIGN -> type(ASSIGN);
 METADATA : 'metadata';
 
+// we need this for EXPLAIN
+FROM_RP : RP -> type(RP), popMode;
+
 // in 8.14 ` were not allowed
 // this has been relaxed in 8.15 since " is used for quoting
 fragment UNQUOTED_SOURCE_PART

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.tokens

@@ -1210,7 +1210,12 @@ public enum Cap {
          *
          * https://github.com/elastic/elasticsearch/issues/129322
          */
-        NO_PLAIN_STRINGS_IN_LITERALS;
+        NO_PLAIN_STRINGS_IN_LITERALS,
+
+        /**
+         * (Re)Added EXPLAIN command
+         */
+        EXPLAIN(Build.current().isSnapshot());
 
         private final boolean enabled;