* FTSEurofirst 300 up 0.9 pct, hits highest level since June 2008* Peripheral banks rally as ECB opens door to June move* Metro and ProSieben rise after updatesBy Francesco CanepaLONDON, May 8 (Reuters) - A key European equity index surged to near six-year highs on Thursday, led by Italian and Spanish banks, after the European Central Bank opened the door to more stimulus measures in June.The pan-