Use a function to determine if an aggregation function is experimental (#15331)

Signed-off-by: Jeanette Tan <jeanette.tan@grafana.com>

promql/parser/lex.go

@@ -68,6 +68,12 @@ func (i ItemType) IsAggregatorWithParam() bool {
 	return i == TOPK || i == BOTTOMK || i == COUNT_VALUES || i == QUANTILE || i == LIMITK || i == LIMIT_RATIO
 }
 
+// IsExperimentalAggregator defines the experimental aggregation functions that are controlled
+// with EnableExperimentalFunctions.
+func (i ItemType) IsExperimentalAggregator() bool {
+	return i == LIMITK || i == LIMIT_RATIO
+}
+
 // IsKeyword returns true if the Item corresponds to a keyword.
 // Returns false otherwise.
 func (i ItemType) IsKeyword() bool { return i > keywordsStart && i < keywordsEnd }

promql/parser/parse.go

@@ -447,8 +447,8 @@ func (p *parser) newAggregateExpr(op Item, modifier, args Node) (ret *AggregateE
 
 	desiredArgs := 1
 	if ret.Op.IsAggregatorWithParam() {
-		if !EnableExperimentalFunctions && (ret.Op == LIMITK || ret.Op == LIMIT_RATIO) {
-			p.addParseErrf(ret.PositionRange(), "limitk() and limit_ratio() are experimental and must be enabled with --enable-feature=promql-experimental-functions")
+		if !EnableExperimentalFunctions && ret.Op.IsExperimentalAggregator() {
+			p.addParseErrf(ret.PositionRange(), "%s() is experimental and must be enabled with --enable-feature=promql-experimental-functions", ret.Op)
 			return
 		}
 		desiredArgs = 2