[ES|QL] Enable the completion command as a tech preview feature (#128948)

Author: afoucret

docs/changelog/128948.yaml

@@ -0,0 +1,6 @@
+pr: 128948
+summary: ES|QL - Add COMPLETION command as a tech preview feature
+area: ES|QL
+type: feature
+issues:
+- 124405

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

@@ -56,10 +56,10 @@ processingCommand
     | mvExpandCommand
     | joinCommand
     | changePointCommand
+    | completionCommand
     // in development
     | {this.isDevVersion()}? inlinestatsCommand
     | {this.isDevVersion()}? lookupCommand
-    | {this.isDevVersion()}? completionCommand
     | {this.isDevVersion()}? insistCommand
     | {this.isDevVersion()}? forkCommand
     | {this.isDevVersion()}? rerankCommand
@@ -308,7 +308,7 @@ rerankCommand
     ;
 
 completionCommand
-    : DEV_COMPLETION prompt=primaryExpression WITH inferenceId=identifierOrParameter (AS targetField=qualifiedName)?
+    : COMPLETION prompt=primaryExpression WITH inferenceId=identifierOrParameter (AS targetField=qualifiedName)?
     ;
 
 sampleCommand

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

@@ -9,6 +9,7 @@ lexer grammar Expression;
 //
 // Expression - used by many commands
 //
+COMPLETION : 'completion'     -> pushMode(EXPRESSION_MODE);
 DISSECT : 'dissect'           -> pushMode(EXPRESSION_MODE);
 EVAL : 'eval'                 -> pushMode(EXPRESSION_MODE);
 GROK : 'grok'                 -> pushMode(EXPRESSION_MODE);
@@ -18,7 +19,6 @@ SORT : 'sort'                 -> pushMode(EXPRESSION_MODE);
 STATS : 'stats'               -> pushMode(EXPRESSION_MODE);
 WHERE : 'where'               -> pushMode(EXPRESSION_MODE);
 
-DEV_COMPLETION : {this.isDevVersion()}? 'completion'   -> pushMode(EXPRESSION_MODE);
 DEV_INLINESTATS : {this.isDevVersion()}? 'inlinestats' -> pushMode(EXPRESSION_MODE);
 DEV_RERANK : {this.isDevVersion()}? 'rerank'           -> pushMode(EXPRESSION_MODE);
 DEV_SAMPLE : {this.isDevVersion()}? 'sample'           -> pushMode(EXPRESSION_MODE);

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

@@ -925,7 +925,7 @@ public enum Cap {
         /**
          * Support for COMPLETION command
          */
-        COMPLETION(Build.current().isSnapshot()),
+        COMPLETION,
 
         /**
          * Allow mixed numeric types in conditional functions - case, greatest and least