re-add missing key method

Signed-off-by: Alex Meijer <ameijer@kubecost.com>
(cherry picked from commit 9e30a15d15f1c83c5a51950a29a1c5440f10eb1b)
Signed-off-by: Alex Meijer <ameijer@kubecost.com>

pkg/costmodel/allocation_types.go

@@ -202,6 +202,11 @@ func (p *pv) minutes() float64 {
 	return p.End.Sub(p.Start).Minutes()
 }
 
+// key returns the pvKey for the calling pvc
+func (p *pv) key() pvKey {
+	return newPVKey(p.Cluster, p.Name)
+}
+
 // lbCost describes the start and end time of a Load Balancer along with cost
 type lbCost struct {
 	TotalCost float64