Any person who shall have been concerned in gaming contrary to this Act, and who shall be examined as a witness before a Magistrate on the trial of any person for a breach of any of the provisions of this Act relating to gaming, and who, upon such examination, shall in the opinion of the Magistrate make true and faithful discovery, to the best of his knowledge, of all things as to which he shall be so examined, shall thereupon receive from the said Magistrate a certificate in writing to that effect, and shall be freed from all prosecutions under this Act for anything done before that time in respect of such gaming.