Fix timestamp() method for vector selector inside paren (#8164)

Signed-off-by: NGanesh Vernekar <cs15btech11018@iith.ac.in>

promql/engine.go

@@ -1027,6 +1027,7 @@ func (ev *evaluator) eval(expr parser.Expr) (parser.Value, storage.Warnings) {
 			// Matrix evaluation always returns the evaluation time,
 			// so this function needs special handling when given
 			// a vector selector.
+			unwrapParenExpr(&e.Args[0])
 			vs, ok := e.Args[0].(*parser.VectorSelector)
 			if ok {
 				return ev.rangeEval(func(v []parser.Value, enh *EvalNodeHelper) (Vector, storage.Warnings) {

promql/testdata/functions.test

@@ -300,6 +300,9 @@ eval instant at 0s timestamp(metric)
 eval instant at 5s timestamp(metric)
   {} 0
 
+eval instant at 5s timestamp(((metric)))
+  {} 0
+
 eval instant at 10s timestamp(metric)
   {} 10