fix: use syntax.MustClone to avoid panics when cloning expressions (#11595)

The clone function in rangemapper has been a persistent source of panics
due to the way stringifying exprs changed the way the original query was
represented. This PR replaces the `clone` function with
`syntax.MustClone`. It clones the expression by actually walking the AST
rather than doing `expr -> string -> parse`. It might still panic but
for a more fundamental reason than because of issues with the string
representation.

pkg/logql/rangemapper.go

@@ -120,7 +120,7 @@ func (m RangeMapper) Parse(expr syntax.Expr) (bool, syntax.Expr, error) {
 // is pushed down to the downstream expression.
 func (m RangeMapper) Map(expr syntax.SampleExpr, vectorAggrPushdown *syntax.VectorAggregationExpr, recorder *downstreamRecorder) (syntax.SampleExpr, error) {
 	// immediately clone the passed expr to avoid mutating the original
-	expr = clone(expr)
+	expr = syntax.MustClone(expr)
 	switch e := expr.(type) {
 	case *syntax.VectorAggregationExpr:
 		return m.mapVectorAggregationExpr(e, recorder)
@@ -277,7 +277,7 @@ func (m RangeMapper) vectorAggrWithRangeDownstreams(expr *syntax.RangeAggregatio
 // appendDownstream adds expression expr with a range interval 'interval' and offset 'offset' to the downstreams list.
 // Returns the updated downstream ConcatSampleExpr.
 func appendDownstream(downstreams *ConcatSampleExpr, expr syntax.SampleExpr, interval time.Duration, offset time.Duration) *ConcatSampleExpr {
-	sampleExpr := clone(expr)
+	sampleExpr := syntax.MustClone(expr)
 	sampleExpr.Walk(func(e syntax.Expr) {
 		switch concrete := e.(type) {
 		case *syntax.RangeAggregationExpr:
@@ -491,17 +491,3 @@ func isSplittableByRange(expr syntax.SampleExpr) bool {
 		return false
 	}
 }
-
-// clone returns a copy of the given sample expression
-// This is needed whenever we want to modify the existing query tree.
-// clone is identical to syntax.Expr.Clone() but with the additional type
-// casting for syntax.SampleExpr.
-func clone(expr syntax.SampleExpr) syntax.SampleExpr {
-	e, err := syntax.ParseSampleExpr(expr.String())
-	if err != nil {
-		panic(
-			errors.Wrapf(err, "error cloning query: %s", expr.String()),
-		)
-	}
-	return e
-}