The boom in office building construction has brought good news - in a backhanded way - to prospective office space renters. That's because in many metropolitan centers there's a definite glut of suitable space. A recent survey of big-city downtown office vacancies indicates a 6 to 9 percent decline in office rents from the prices asked last year. There is a good deal of stimulation going on to induce renters to ''move in'' with a variety of incentives, according to the survey. These include several months' free rent, flat long-term agreements (no rent rises), moving expenses paid by the landlord, and cash rebates.