add option to enable holt winters (#179)

Signed-off-by: yeya24 <benye@amazon.com>

Author: yeya24

opts.go

@@ -76,6 +76,10 @@ func init() {
 	}
 
 	for _, f := range parser.Functions {
+		// holt_winters is excluded by default; use WithEnableHoltWinters(true) for older backends.
+		if f.Name == "holt_winters" {
+			continue
+		}
 		// Ignore experimental functions for now.
 		if !f.Experimental {
 			defaultSupportedFuncs = append(defaultSupportedFuncs, f)
@@ -94,6 +98,7 @@ type options struct {
 	enableOffset                      bool
 	enableAtModifier                  bool
 	enableVectorMatching              bool
+	enableHoltWinters                 bool // holt_winters not in Prometheus 3.x; enable for older backends
 	enableExperimentalPromQLFunctions bool
 	atModifierMaxTimestamp            int64
 
@@ -119,6 +124,10 @@ func (o *options) applyDefaults() {
 		o.enabledFuncs = defaultSupportedFuncs
 	}
 
+	if o.enableHoltWinters && parser.Functions["holt_winters"] != nil {
+		o.enabledFuncs = append(o.enabledFuncs, parser.Functions["holt_winters"])
+	}
+
 	if o.enableExperimentalPromQLFunctions {
 		o.enabledAggrs = append(o.enabledAggrs, experimentalPromQLAggrs...)
 		o.enabledFuncs = append(o.enabledFuncs, experimentalSupportedFuncs...)
@@ -168,6 +177,15 @@ func WithEnableVectorMatching(enableVectorMatching bool) Option {
 	})
 }
 
+// WithEnableHoltWinters enables generation of holt_winters() in queries.
+// Disabled by default because Prometheus 3.x replaced it with double_exponential_smoothing;
+// enable only when fuzzing older backends that still support holt_winters.
+func WithEnableHoltWinters(enable bool) Option {
+	return optionFunc(func(o *options) {
+		o.enableHoltWinters = enable
+	})
+}
+
 func WithEnableExperimentalPromQLFunctions(enableExperimentalPromQLFunctions bool) Option {
 	return optionFunc(func(o *options) {
 		o.enableExperimentalPromQLFunctions = enableExperimentalPromQLFunctions