promql: put holt_winters behind experimental feature flag

Signed-off-by: Jan Fajerski <jfajersk@redhat.com>

docs/querying/functions.md

@@ -382,6 +382,8 @@ variance of observations in a native histogram.
 
 ## `holt_winters()`
 
+**This function has to be enabled via the [feature flag](../feature_flags.md#experimental-promql-functions) `--enable-feature=promql-experimental-functions`.**
+
 `holt_winters(v range-vector, sf scalar, tf scalar)` produces a smoothed value
 for time series based on the range in `v`. The lower the smoothing factor `sf`,
 the more importance is given to old data. The higher the trend factor `tf`, the

promql/parser/functions.go

@@ -206,6 +206,7 @@ var Functions = map[string]*Function{
 		Name:         "holt_winters",
 		ArgTypes:     []ValueType{ValueTypeMatrix, ValueTypeScalar, ValueTypeScalar},
 		ReturnType:   ValueTypeVector,
+		Experimental: true,
 	},
 	"hour": {
 		Name:       "hour",