METHOD OF AND APPARATUS FOR MEASUREMENT OF ELECTROCHEMICAL PARAMETERS OF METAL UNDERGROUND STRUCTURES,ESPECIALLY FOR MEASUREMENT OF ELECTROCHEMICAL PARAMETERS OF GAS PIPINGS LOCATED IN STRAY CURRENTS OPERATION ZONE PARAMETROV METALLICHESKIKH PODZEMNYKH KONSTRUKCIJJ,A OSOBENNO DLJA IZMERENIJA EHLEKTROKHIMICHESKIKH PARAMETROV GAZOPROVODOV,NAKHODJAHHIKHSJA V ZONE VOZDEJJSTVIJA BLUZHDAJUHHIKJ TOKOV