Expression: Fix a bug of the display name of the threshold expression result (#85912)

* replace ReferenceVar with RefID when creating the math command for the ThresholdCommand

* Fix the Hysteresis test

pkg/expr/hysteresis_test.go

@@ -15,7 +15,7 @@ import (
 
 func TestHysteresisExecute(t *testing.T) {
 	number := func(label string, value float64) mathexp.Number {
-		n := mathexp.NewNumber("A", data.Labels{"label": label})
+		n := mathexp.NewNumber("B", data.Labels{"label": label})
 		n.SetValue(&value)
 		return n
 	}

pkg/expr/threshold.go

@@ -120,7 +120,7 @@ func (tc *ThresholdCommand) Execute(ctx context.Context, now time.Time, vars mat
 		return mathexp.Results{}, err
 	}
 
-	mathCommand, err := NewMathCommand(tc.ReferenceVar, mathExpression)
+	mathCommand, err := NewMathCommand(tc.RefID, mathExpression)
 	if err != nil {
 		return mathexp.Results{}, err
 	}