A covenant by an assistant, servant or agent not to compete with his or her employer or principal during the term of the employment or agency, or after the termination of that employment or agency, within a specified territory and during a specified time is lawful and enforceable only if the restrictions imposed are reasonably necessary for the protection of the employer or principal. Any covenant, described in this section, imposing an unreasonable restraint is illegal, void and unenforceable even as to any part of the covenant or performance that would be a reasonable restraint.